THIS INVENTION relates to telephone billing. In particular, the invention relates to a method of billing for use of a telephone. The invention also relates to a SIM card for connection to a telephone, and to a telephone assembly which includes the SIM card. The invention extends to a set of SIM card executable instructions.

Pre-paid mobile telephone billing is currently managed by a mobile network operator. The equipment required to operate a payphone business using a cellular telephone operating on pre-paid airtime is bulky and costly, and often impractical.

The invention provides a method of billing for use of a telephone which has an electronic identification module, the method including storing on the identification module account information relating to a telephone account for use of the telephone, and automatically adjusting the account information in response to billable use of the telephone.

In other words, the invention provides a telephone billing method in which billing is managed by an electronic identification module which is connectable to a telephone. The identification module may be a Subscriber Identification module (SIM) card which is installed in a mobile telephone, such as a cellular telephone. The SIM card may comprise integrated circuitry which provides an electronic processor and memory means, so that the SIM card is programmable. In a preferred embodiment, the SIM card is programmable by use of SIM application Toolkit.

The term SIM card is meant to include any electronic module which is removably connectable to a telephone device for uniquely identifying itself to a telephone network.

The method may thus include providing a set of computer readable instructions on the SIM card for enabling the SIM card to manage the account associated with the SIM card, when the set of instructions is executed.

Storing account information on the SIM card may include storing on the SIM card an account balance for the account managed by the SIM card.

The method may also include storing on the SIM card transaction information about individual telephonic transactions performed for the account on the ' SIM card. Such transaction information may include the duration of a particular billable event or telephone call, as well as the cost of each billable event.

The account which is maintained on the SIM card may be a prepaid account, the SIM card thus being arranged to permit the making of telephonic transactions only if the account balance stored on the on SIM card is positive. The account information may thus include a balance of available credit, which may be in the format of a monetary value. The method may include automatically increasing or decreasing the account balance on the SIM card in response to additions and deductions to the account.

The account which is managed by the SIM card may be a payphone account, the method including calculating the cost of each billable event by use of the SIM card and adjusting an account balance for that call accordingly. Preferably, the method includes specifying a cost limit for each billable event, and continually adjusting the available funds in the account balance for that event during progress of the event, the event preferably being ended automatically when the available funds in the event account balance are depleted.

Instead, or preferably in addition, the method may include managing a network account by use of the SIM card, the network account having a network balance which is automatically adjusted during use of the telephone. It will be appreciated that the method defined above is particularly suited to use in providing a payphone service, as an owner, vendor or operator can permit customers to use the telephone, the cost of the billable events being calculated by the SIM card and an event account being managed for each billable event. Conveniently, the method may include permitting adjustment of billing rules on the event account, for use by customers.

It will be appreciated that events which result in a reduction of the account balance on the SIM card typically include calls or messages which are made on the telephone against the account. The method may thus include continuously adjusting the account balance during the making of a telephone call, and automatically terminating the telephone call if the account balance is depleted.

Use of the telephone on credit against the account may be permitted, so that telephone transactions are allowed when the account balance is negative.

The method may include permitting a user to set a maximum charge and/or duration for a particular telephone call; tracking the charge and duration of the telephone call while it is in progress; and automatically terminating the telephone call when the maximum charge or duration is reached.

The method may include receiving an update communication from a network operator, the update communication including information relating to the account which is maintained by the SJM card, and automatically changing the account information stored on the SIM card to reflect the update communication, if required. Preferably, such update communications between the network operator and the telephone are via an essentially wireless telephone network, such as a cellular telephone network, on which the telephone operates.

The update communication may include information indicative of a deposit made against the account, the method including automatically increasing the account balance stored on the SIM card by an amount represented in the update communication. As mentioned above, the update communication is typically sent by an authorised sender such as, for example, a service provider linked to a bank. In use, a network subscriber will thus typically buy a certain amount of airtime for the SIM card account by paying a deposit at a bank or other authorised deposit receiving facility. In response, an update communication is automatically sent to the SIM card to authorise a corresponding increase in the SIM card account. It will be appreciated that all such communications with the SIM card may be encrypted, to inhibit fraudulent tampering with the account information on the SIM card. Preferably, the encryption is at network operator level.

When a subscriber has paid a deposit for increasing the SIM card account, the update communication may be sent to the SIM card and the account balance on the SIM card updated in response thereto. The reception of the update communication and changing of the account information on the SIM card may occur without displaying notification thereof on the telephone. The method may include receiving a notification message from a wireless telephone network operator in respect of the update communication, and displaying the notification message on the telephone, the update communication being in the form of a telephonic text message.

The method may include storing on the SIM card billing information comprising different billing rules for different billable events, each billing rule including billing parameters for the associated billable event. Such billing information may include information relating to billing of audio or data communications like, for example, voice calls or SMSs. The method may include billing voice calls as closely as possible to accepted telecommunications regulations by applying a split billing sliding scale. The SIM card may thus be programmed with separate billing rules for: local mobile network voice calls; non-local mobile network voice calls; local public landline network voice calls; international voice calls; free or reserved voice calls; and SMSs and other data communications.

The method may include keeping track by the SIM card of the duration of any telephone call, and using the duration of the telephone call in combination with the billing parameters associated with the particular billable event to calculate the cost of the call.

The method may include updating the billing information on the SIM card in response to. receiving an update communication which contains information indicative of a change in billing information. It will be appreciated that billing rates vary from time to time, and that the billing information on the SIM card should preferably correspond to current billing rules applied by the telephone network.

Each billing rule may include billing parameters which determine how billing is done for the associated billable event. The various billing rules may have common parameters, which typically include: an initial time unit; a cost of the initial time unit; a subsequent time unit; a cost of the subsequent time unit; number prefixes associated with different billing rules; and a data cost.

The initial time unit may be measured in seconds and may define the duration of the first unit of the call. The cost of the initial time unit may be charged in response to the voice call being connected to a receiving entity. The subsequent time unit may be measured in seconds and may define the duration of time units following the first unit of the call.

The billing information may include a plurality of number prefixes which are associated with respective billable events, the type of billable event which is performed automatically being identified by reading the number prefix of a call, billing parameters of the identified type of billable event being applied to the call. Each number prefix preferably comprises five digits or four digits and a '+' sign. The cost of time units may be recorded in the lowest or least common denominator of currency of the country concerned. In South Africa, for example, the cost may therefore be recorded in cents. The number prefixes may allow for different telephone number ranges. Thus the cost associated with a billing rule may vary according the number prefix with which the dialled telephone number corresponds.

The method may include automatically identifying the type of billable event by reading the number prefix of a call, and applying the associated billing parameters to the billable event. The appropriate billing rule may thus be automatically applied to its corresponding call type to determine a cost associated with that call.

The method may include displaying account information and/or transaction information on a telephone to which the SIM card is connected. The account information and/or the transaction information may be made available on a selection menu displayed on a display, means of the telephone. In use, a user of the telephone will thus be able to view the cost and/or duration of any call or other billable event immediately after making the call. The user will also be able to view on the display screen the current account balance on the SIM card.

It will be appreciated that permitting immediate viewing of call costs facilitates use of a telephone connected to the SIM card as a payphone.

The method may include keeping track, at network level, of use of the SIM card, to calculate a current account balance associated with the SIM card, the account balance stored on the SIM card being automatically periodically compared with the calculated account balance at network level, and the account being flagged if there is a substantial discrepancy between the calculated account balance at network level and the account balance stored on the SIM card.

The invention extends to a SIM card which is configured for managing billing for use of a telephone to which the SIM card is, in use, connected.

The SIM card may be programmable, the SIM card having a memory on which is stored: account information relating to a telephone account which is managed by the SIM card; and a set of instructions for automatically adjusting the account information in response to billable use of a telephone to which the SIM card is connected. In other words, the SIM card may be provided with a software application or program for management of an airtime account on the SIM card. The SIM card may conveniently be programmed with SIM application Toolkit (STK).

Differently defined, the invention extends to a telephone SIM card which includes a set of instructions for managing billing for use of a telephone to which the SIM card is, in use, connected.

The SIM card may be programmed to store thereon an account balance for the account managed by the SIM card, and automatically to increase or decrease the account balance in response to additions to or deductions from the account.

The SIM card may be programmed to permit a user to set a maximum charge and/or duration for a telephone call, the SIM card being arranged continuously to keep track of the charge and/or duration of a telephone call during its progress, and automatically to terminate the telephone call when that telephone call reaches its pre¬ set maximum charge and/or duration.

The SIM card may be arranged to manage a payphone account or event account by calculating the cost of each billable event by use of the SIM card and to adjust an event account balance accordingly. Preferably, the SIM card is configured to permit a use to a cost limit or maximum charge for each billable event, and continually to adjust the available funds in the account balance for that event during progress of the event, the SIM card typically being arranged to end a billable event automatically when the available funds in the event account balance are depleted. Instead, or preferably in addition, the SIM card may be arranged to manage a network account by use of the SIM card, the network account having a network balance which is automatically adjusted during use of the telephone. In such case, the SIM card may be arranged to manage and store information regarding a network account and an event account. It will be appreciated that the SIM card defined above is particularly suited to use in providing a payphone service, as an owner, vendor or operator can permit customers to use the telephone against the event account, the cost of billable events being calculated by the SIM card and an event account being managed for each billable event. Conveniently, the SIM card may be arranged to permit adjustment of billing rules for charging against the event account.

The SIM card may be programmed to generate and store transaction information about individual telephonic transactions performed for the account on the SIM card, the transaction information including the cost and/or duration of respective billable events.

The SIM card may be arranged to manage a prepaid airtime account, the SIM card being programmed to check the account balance upon attempted initialisation of a billable event on a telephone connected to the SIM card, and to permit performance of the billable event only if the account balance is positive.

The SIM card may also be arranged to increase the account balance in response to reception of an update or recharge communication from a network operator. In addition, the SIM card may be arranged to receive and to display on the telephone a message, typically an SMS message, advising a user that a particular amount has been added to the account balance.

The SIM card may be configured for automatically terminating a telephone call which is in progress, when the account balance reaches a pre-determined minimum amount. This minimum amount will be zero in the case of a pre-paid account.

The SIM card may have stored thereon billing information comprising different billing parameters for different billable events. The billing information may include billing parameters for billable events selected from the group comprising: local mobile network voice calls; non-local mobile network voice calls; local public landline network voice calls; international voice calls; free or reserved voice calls; and SMSs and other data communications.

The memory of the SIM card may therefore conceptually be divided into account memory in which account information is stored, and billing memory in which billing information is stored.

The SIM card may be arranged for updating the billing information thereon in response to reception of an update communication from a network operator.

The SIM card may be programmed to permit selective display of the account information and the transaction information on a telephone to which the SIM card is connected. The invention extends further to a telephone assembly which includes: a telephone; and a SIM card as defined above, which is connected to the telephone for directing operation of the telephone.

The telephone may be a mobile telephone, preferably being a cellular telephone (further referred to as a cellphone). The telephone typically operates on the GSM standard.

The invention extends further to a set of SIM card executable instructions for enabling a SIM card to perform a method as described above, when the set of instructions is executed by the SIM card.

An embodiment of the invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a schematic diagram of a system for telephone billing, the system including a telephone which has a SIM card in accordance with the invention; Figure 2 is a flow diagram of method steps performed by the SIM card of Figure 1, to send an SMS via the telephone; Figure 3 is a flow diagram of method steps performed by the SIM card of Figure 1 to initiate a voice call; Figure 4 is a flow diagram of method steps performed by the SlM card of Figure 1 during the first time unit of a voice call made with the telephone; Figure 5 is a flow diagram of method steps performed by the SIM card of Figure 1 during subsequent time units of a voice call made with the telephone; Figure 6 is a flow diagram of method steps performed by the SIM card of Figure 1 for the billing of subsequent time units of a voice call made with the telephone; and Figure 7 is a flow diagram of method steps performed by the SIM card of Figure 1 to terminate a voice call.

In Figure 1 , reference numeral 10 generally indicates a system for performing a method of telephone billing in accordance with the invention, the telephone billing being managed by a SIM card 14 in accordance with the invention.

The system 10 includes a cellphone 12 which is configured for use on a GSM-standard cellular telephone network 20. The network 20 includes network operators (not shown) which provide support services related to use of cellphones 12 over the network 20. It will be appreciated that the system 10 includes a multitude of cellphones 12 which can communicate over the network 20, but that only one of the cellphones 12 are shown, for clarity of illustration.

The cellphone 12 contains a SIM card 14 which serves to identify itself to the network 20, for controlling whether or not calls can be made by the cellphone 12 via the network 20. Such SIM cards are usually associated with particular subscribers, so that the network operator can establish for whose account the cellphone 12 is used. The SIM card 14 is programmable so that the cellphone 12 and the SIM card 14 communicate in conventional fashion. It will be appreciated that the SIM card 14 is configured to initiate and control communication with an electronic processor and connected equipment of the cellphone 12, so that the SIM card 14 can control to an extent the applications run on the cellphone 12 and the information displayed on a display screen of the cellphone 12.

The SIM card 14 has been pre-programmed with a set of instructions for managing an airtime account on the SIM card 14. The SIM card 14 thus includes a software application or program, schematically represented by numeral 14.1. In this example, the SIM card 14 is programmed with SIM application Toolkit (STK).

The SIM card 14 has a memory arrangement which is configured to store account information in an account memory 14.2 and transaction information in a billing memory 14.3. The instructions of the program 14.1 relate to the processing of account information and transaction information. It should be appreciated that separate representation of the account memory 14.2 and the billing memory 14.3 is merely schematic, and that the account memory 14.2 and the billing memory 14.3 may be integral with the program 14.1.

The account memory 14.2 on the SIM card 14 includes a pre-paid account balance, which indicates how much credit a user has with which to make telephone calls or send other communications, such as SMSs. It should be appreciated that although the SIM card 14 is programmed in this example to manage a pre-paid airtime account, the SIM card 14 can, in other embodiments, be arranged for managing any type of telephone account. The program 14.1 is arranged to increase the account balance in response to receiving an update or recharge communication from a network operator that a deposit has been made against the account managed on the SIM card 14. A user of the telephone 12 may choose to increase the balance by any convenient method, for example, by depositing money into an associated bank account (not shown). Once the user has chosen to increase the balance and taken the necessary steps, the cellphone 12 receives an update communication via the network 20 indicating an amount by which the balance must be increased.

The SIM card 14 then automatically, under the instructions of the program 14.1 , adds the indicated amount to the balance, and the resultant balance is stored in the account memory 14.2 on the SlM card 14. The process will be repeated for future balance increases.

The update communication is received and processed by the telephone 12 and the SIM card 14 without the user's knowledge. However, to alert the user to an increase in his account, an SMS is automatically sent by the network operator to the telephone 14. This text message informs the user that the balance has been increased and by what amount it has increased.

The program 14.1 is further arranged to cause automatic reduction in the account balance in response to the performance of a billable event, such as a voice call or an SMS, on the telephone 12. The SIM card 14 thus calculates the cost of any call, while the call is being performed, and immediately adjusts the account balance. The cost of each call is calculated with reference to billing information which forms part of the transaction information stored on the SIM card in the billing memory 14.3. The billing information determines how charges are calculated and at what rates.

In this example, the billing information includes six billing rules. These six rules are respectively associated with: local mobile network voice calls; non-local mobile network voice calls; local public landline network voice calls; international voice calls; free or reserved voice calls; and SMSs and other data communications.

Each billing rule contains common parameters that may be assigned different values. The parameters are: an initial time unit; a cost of the initial time unit; a subsequent time unit; a cost of the subsequent time unit; number prefixes; and a data cost.

All these parameters are assigned values depending on how the call is to be billed. The initial time unit is measured in seconds and defines duration of the first unit of the call. If the initial time unit is 30 seconds, the first 30 seconds of the call are included in the initial time unit. A cost is associated with the initial time unit, in this example being 180 cents. This means a call of 30 seconds or shorter is billed at 180 cents. The cost of the initial time unit is charged in response an indication that the call is connected to a receiving entity.

The subsequent time unit defines all units of time other than the initial unit. Therefore a subsequent time unit defines a second unit, a third unit, a fourth unit, and so forth. If a subsequent time unit is, for example, 60 seconds, the second unit of time will be from 30 seconds after the call was connected to 90 seconds after to call was connected. The subsequent time unit cost, typically being different from the cost of the initial time unit, is charged for each subsequent time unit. In this example, the subsequent time unit cost is 90 cents. This means a call of between 31 and 90 seconds will be billed at 270 cents (being the sum of the initial time unit, 180 cents, and one subsequent time unit, 90 cents).

The cost of time units is recorded in the lowest common denominator, or least common denominator, of currency of the country concerned. In South Africa, for example, the cost will therefore be recorded in cents. For example, if the cost of a particular unit is R1.44, the system 10 will record the cost as 144.

The SIM card 14 is thus programmed to calculate the total amount at which a call is billed with the formula: cost of initial time unit + cost of subsequent time unit * number of subsequent time units. The system 10 is programmed to bill voice calls as closely as possible to accepted telecommunications regulations by applying a split billing sliding scale principle known as 'initial call duration cost accumulating subsequent call duration cost'.

The respective prefixes correspond to the format of dialling codes, and therefore contain either five digits or four digits and a '+' sign. Different billing rules will include different number prefixes. The billing rule for local mobile network voice calls, for example, contains number prefixes that correspond with mobile network operators in the location where the cellphone 12 is used. The billing rule governing international voice calls includes international dialling prefixes, for example 09264 (the international dialling code from South Africa to Namibia). The SIM card 14 is programmed to determine when an international call is being made, and will automatically charge in accordance with the corresponding billing rule, which will thus automatically bill the call at higher rate than for a local call. Ideally any particular number prefix is associated with only one particular billing rule.

In a similar manner, the data cost parameter, which forms part of the billing information stored on the billing memory 14.3, is used to bill for data communications, such as SMSs.

The program 14.1 on the SIM card 14 is further arranged to cause display of account information and transaction information on the display screen of the cellphone 12. In this example, the information is accessible via a selection menu displayed on the screen, the user being able to select, desired items for display by operation of the cellphone's keypad. The user will thus, at any time, be able to select for viewing the current account details, which includes the latest account balance. The user will also be able to view the transaction details, particularly the duration and the cost of calls made.

Figures 2 to 7 show, schematically, the process steps performed by the SIM card 14, in use, when the telephone 12 is used. Figure 2 is a flow diagram of a sub-program for sending an SMS. To send and SMS the user, in conventional fashion, keys the text message in on the keypad of the cellphone 12 and attempts to send the SMS. The SIM card 14 is programmed to choose the appropriate billing rule from the billing memory 14.3, which will be the rule governing SMSs and other data costs. The only parameter that is pertinent to this rule is the data cost, being the cost of a single data communication. An SMS send event is thus initiated, at block 31 , after which the SIM card 14 compares, at block 32, the balance in the account information 14.2 with the data cost. If the balance is greater than or equal to the data cost, the user has sufficient credit to send an SMS. The data cost is then deducted, at block 33, from the account balance and the resultant balance is stored in the account memory 14.2 on the SIM card 14. The SMS is allowed, at block 34, and is sent in conventional fashion. The SMS send event, at block 31 , ends, at block 36.

If, however, the available balance is smaller than the data cost, the user does not have sufficient credit to send an SMS. The SMS attempt is barred, at block 35, and the SMS send event ends, at block 36.

Figure 3 is a flow diagram of a sub-program for initiating a voice call. To make a call, the user, in conventional fashion, keys in the number to dial, or selects it from a list. The user enters an amount (further referred to a change amount) which represents the maximum amount for the cost of the call. The user then attempts to initiate a voice call, at block 39. The SIM 14 card determines, at block 40, whether or not it has received on override command. If it has received such a command, at branch 40.1, it allows the call attempt to proceed, at block 41 , and ends the sub-program at block 80. If there was no override command, at branch 40.2, the SIM card 14 determines, at block 42, whether or not there is already a call in progress. If there is a call in progress, at branch 42.1 , the call attempt is blocked, at block 43, and the sub¬ program ends at block 80.

If there is not a call in progress, at branch 42.2, the SIM card 14 determines, at block 44, whether or not it has received a USSD (Unstructured Supplementary Service Data) command. If it has received such a command, at branch 44.1, it allows the call attempt to proceed, at block 45, and ends the sub-program at block 80.

If there was no USSD command, at branch 44.2, the SIM card 14 determines, at block 46, whether or not the dialled number was found. If the dialled number was not found, at branch 46.1 , the call attempt is blocked, at block 47, and ends the sub-program, at block 80. If the dialled number is found, at branch 46.2, the SIM card 14 converts the dialled number to an ASCII (American Standard Code for Information Interchange) format, at block 48. Variables representing the total charge and total duration of the call are assigned the value of zero respectively, at block 50. The SIM card 14 determines, at block 52, whether or not the change amount in the account memory 14.2 is equal to zero. At any stage, the change amount reflects the remainder of the amount set by the user before he initiated the voice call. If the change amount is equal to zero, at branch 52.1, it means that the user failed to set an amount, and the screen of the cellphone 12 displays the error message 'Please set an amount first' for four seconds, at block 54. It is to be appreciated that any convenient duration for the error message display could be used, four seconds being a duration which allows the user to read the message but does not freeze the cellphone 12 for too long. The call attempt is blocked, at block 56, and the sub-program comes to an end, at block 80.

If the change amount is not equal to zero, at branch 52.2, the user has entered a maximum amount for the call, and the SIM card 14 compares, at block 58, the number prefix of the dialled number with all the number prefixes in all the billing rules of the billing memory 14.3. Typically, an operator of a payphone business receives a cash deposit from a customer and then sets the maximum charge or change amount on the telephone 12 to equal the deposit. During a call, the change amount is continually decremented until the change amount is depleted. The SIM card 14 selects the billing rule associated with the first number prefix which matches the number prefix of the dialled number. That billing rule is then selected as the billing rule that is applicable to the voice call. If an appropriate billing rule, at block 60, was not found, at branch 60.2, the call attempt is blocked, at block 56, and the sub-program comes to an end, at block 80. In other words, if no match was found, the number prefix does not appear in any billing rule and hence is not an allowed number. If, however, a match for the prefix is found, at branch 60.2, it means that the number prefix of the dialled number does occur in a number prefix of a billing rule. For example, if a user in South Africa dials a number in Namibia, the system 10 is programmed to compare the number prefix, which will be 09264. A match may be found, depending on the configuration, in the international voice calls billing rule, and that rule would be applicable. The parameters associated with the applicable billing rule will be the parameters applied to the voice call.

The SIM card 14 compares, at block 62, the change amount with the cost of the first time unit associated with the applicable billing rule. If the change amount is smaller than the cost of the first time unit, at branch 62.1 , the screen of the cellphone 12 displays the error message 'Please set an amount first' for four seconds, at block 64, blocks the call attempt, at block 66, and the sub-program comes to an end, at block 80.

If the change amount is greater than or equal to the cost of the first time unit, then the SIM card 14 compares, at block 68, the balance in the account information 14.2 with the cost of the first time unit associated with the applicable billing rule. If the balance is smaller than the cost of the first time unit, at branch 68.1 , the user has insufficient credit to initiate a voice call. The screen of the cellphone 12 displays the error message 'Low balance' for four seconds, at block 70, and the call attempt is blocked, at block 66, and the sub-program comes to an end, at block 80.

If, however, the balance is greater than or equal to the cost of the first time unit, at branch 68.2, the user has enough credit to initiate the call, and the dialled number is then compared to a list of barred numbers in the billing memory 14.3, at block 74. If the dialled number corresponds to a barred number, at branch 74.1 , the screen of the cellphone 12 displays the error message 'Barred number' for four seconds, at block 76, and the call attempt is blocked, at block 66, and the sub-program comes to an end, at block 80. If the dialled number does not correspond to a barred number, at branch 74.2, the call attempt is allowed to proceed, at block 78, and the sub-program ends at block 80. If1 at any stage, the call attempt is blocked, the process ends when the current sub-program ends. If however, to call attempt was allowed, the process continues to the next sub-program.

Figure 4 shows a flow diagram of a sub-program for calculating the call costs of a voice call during the first time unit. The sub-program is automatically executed after the sub-program of Figure 3 if the call attempt was allowed to proceed and not blocked. The SlM card 14 waits for the call to be connected, at block 82. If the call did not connect, at branch 82.1 , the call ends, at block 86.

If1 however, the call did connect, at branch 82.2, a timer is assigned the same value as the initial time unit, at block 83, and the timer is started, at block 84. The term timer in this context is understood to mean a variable that counts down at a set rate, starting at a defined number and ending at zero. The cost of the first time unit is added to the variable representing the total charge, at block 85. It will be appreciated that because the variable representing the total charge was zero, said variable will now be equal to the cost of the first time unit. The SIM card 14 is programmed to wait for the timer to reach zero. When the timer does reach zero, the first time unit has ended, and the sub-program terminates, at block 86. Figure 5 and Figure 6 are flow diagrams of sub-programs for calculating the cost of a voice call during subsequent time units. The sub-program of Figure 5 is automatically executed, at block 87, when the timer which was started in the previous sub-program reaches zero.

The SIM card 14 determines, at block 88, whether or note the current iteration of the sub-program is a billing event in which the duration of the event determines its cost. If it is not, at branch 88.1, a message is displayed on the screen of the cellphone 12, at block 89, the message being selected from a message indication set. The message indication set includes the following messages: 'Low balance1; 'Number barred'; 'No number found'; 'Consecutive call not allowed'; and 'Please set amount'.

If, however, the current iteration is a timer billing event, at branch 88.2, it is determined, at block 90, whether or not the remaining balance in the account memory 14.2 is greater than or equal to the cost of a subsequent call unit. If the balance is not greater than or equal to the cost of a subsequent call unit, at branch 90.1, it means that there is not enough credit remaining to pay for the next call unit, and the SIM card 14 issues the AT+CHUP command (a standard hang-up command) to the cellphone 12, to terminate the call. If the balance is greater than or equal to the cost of a subsequent call unit, at branch 90.2, it is determined, at block 92, whether or not the change amount is greater than or equal to the cost of a subsequent call unit. If it is not, at branch 92.1 , the user has spent all the money which he wanted to spend on the call, and the AT+CHUP command is issued to the cellphone 14 to terminate the call. If it is greater than the cost of a subsequent time unit, at branch 92.2, the timer is set to the subsequent call unit duration, at block 94.

The cost of a subsequent call unit is then charged, at block 96 (see Figure 6 for more detail).

At block 98, the SIM card 14 issues the AT+CPAS command to the cellphone 12 to determine, at block 100, whether the cellphone 12 is on the hook or whether it is off the hook. If the cellphone 12 is on the hook, at branch 100.1 , it means that the call has ended, and the sub-program ends at block 104. If the cellphone 12 is off the hook, at branch 100.2, the SIM card 14 issues the AT+CHUP command to terminate the phone call, at block 102, and the sub-program ends, at block 104.

Referring now to Figure 6, the flow diagram shows a sub-program for charging the cost of a subsequent time unit. The sub-program is initiated by block 96 of the sub-program of Figure 5, and starts at block 110. It is determined, at block 112, whether or not the total call duration is greater than a duration threshold. The duration threshold is a timer used to track the time spent on a call and thereby monitor the duration of the call. If it is not, at branch 112.1 , the duration of a subsequent call unit is added to the total call duration, at block 122. If the total call duration is not greater than the duration threshold, at branch 112.2, the cost of a subsequent time unit is added to the total call charge, at block 114, subtracted from the balance, at block 116, and subtracted from the change amount, at block 118. The duration of a subsequent call unit is then added to the duration threshold, at block 120, and added to the total call duration, at block 122. The sub-program then ends, at block 124.

It is to be appreciated that the sub-program of Figure 6 will have as many iterations as there are subsequent call units.

Figure 7 shows a flow diagram of a sub-program for terminating a voice call, the sub-program being automatically executed, at block 130, when the call is ended. At block 132, the SIM card 14 determines whether or not the call did indeed disconnect. If it did not, at branch 132.1 , the sub-program ends. If it did, at branch 132.2, the screen of the cellphone 12 displays, at block 134, the message 'Last call charge: x', where x is the charge of the last call, for four seconds, and sets call ended state flags

There are three call ended state flags which respectively are called 'call control in progress', 'timer do count', and 'continue call make' and are used to track the status of the call. The 'call control in progress' flag indicates whether the user may view the menu, or whether a call is in progress in which case viewing of the menu is disabled. This avoids the user interfering with the billing process of the current call. The 'timer do count' flag indicates whether the timer on the next timer event should restart in preparation for that timer event or whether the timer should not start another timer event. The 'continue call make' flag indicates whether a call attempt may be allowed or whether to block the call attempt and bar the call.

It is determined whether or not a certain trigger level is reached, at block 136. The trigger level is a preset variable which indicates when to send an SMS to a predefined telephone number to indicate that the value of the transactions (that is, voice calls and SMSs) on the cellphone has reached that trigger level.

If the trigger level was not reached, at branch 136.1, the sub-program ends, at block 140. If the trigger level was reached, at branch 136.2, an SMS is sent, at block 138, to a predefined number as a notification. The sub-program then terminates, at block 140.

The SIM card 14 provides the cellphone 12 with menu options (not shown) for allowing the user to view information relating to the pre-paid airtime account at any time. To view information, an option entitled 'Sharedfone' on the menu of the cellphone 12 is selected in conventional fashion. Selecting the 'Sharedfone' option displays four sub-options that the user may select.

The first sub-option is entitled 'Call charge'. When selected, this sub- option displays information relating to calls that the user has made, comprising: . the total charge of a call; the duration of that call; and the number that was dialled.

The second sub-option is entitled 'Change'. When selected, this sub- option displays the amount remaining from the initial amount entered before the phone call was made or the SMS was sent, the amount being the difference between the initial amount entered and the cost of the phone call made or the SMS sent.

The third sub-option is entitled 'Balance'. When selected, this sub-option displays the current balance available to the

The fourth sub-option is entitled 'Purse Manager'. An owner PIN (personal identification number) must be entered into the phone to access this option. Once the owner PIN is entered, the user may view a further sub-menu which provides five further sub-options (referred to as purse manager sub-options).

The first purse manager sub-option is entitled 'Sales total'. When selected, this purse manager sub-option displays information relating to units which have been used.

The second purse manager sub-option is entitled 'Clear sales total'. When selected, this purse manager sub-option resets to zero the running total of units which have been used. The third purse manager sub-option is entitled 'Set postpay'. When selected, this purse manager sub-option sets the mode of operation to 'postpay' model. The 'postpay' model does not require the user to set a change amount before a call and will deduct the cost of the call from the balance in the same manner as if a change amount had been set first. This option is convenient for allowing users to make calls on credit.

The fourth purse manager sub-option is entitled 'Change PIN'. This purse manager sub-option is used by the user to change the owner PIN. When selected, the user is prompted to enter the current owner PIN, and, if correctly entered, the user is prompted to enter a new owner PIN and is then prompted to confirm the new owner PIN by entering it again. If the new owner PIN is correctly confirmed, it will be set as the owner PIN.

The fifth purse manager sub-option is entitled 'Set profits'. When selected, this purse manager sub-option is used by the user to change the billing cost and time duration of categories of billing rates. Each billing category defines a set of dialled number prefixes and the time duration and cost associated with calls made to phone numbers starting with one of the dialled number prefixes.

It is to be appreciated that the use of the telephone 12 is billed by the SIM card 14 at network rates. However, the operator or owner of the telephone can set a rate or duration of time units at which the user's customers are charged. The applicant believes that this invention provides an advantageous method of telephone billing, especially in the payphone industry. The embodiment of the invention described above allows virtual prepaid networks to operate on existing GSM networks. The cost of entry of new businesses is reduced as no complex billing and administrative infrastructures are required.

An airtime vendor can thus sell airtime on the cellphone 12, receiving payment from clients for use of the cellphone 12. As the cost and duration of each transaction is immediately displayed, there is no need for the vendor to keep track of the duration of the calls and to calculate the cost of individual calls. The vendor also has the option of setting a maximum cost of a telephone call, so that the call automatically ends when the pre-set cost is reached. It will be appreciated that the SIM card's program 14 is arranged to permit adjustment by a vendor of the call rate at which airtime is sold on the cellphone 12 as described above, so that the vendor's profit margin will be determined by the difference between the cost of airtime paid to the network, and the price at which airtime is sold. In such case, the maximum cost of the call, which is set by the vendor, is calculated at the cost rate adjusted by the vendor, while the account balance is adjusted at network rates.

The need for vouchers is eliminated as recharge deposits can be made directly to a bank account. A more effective means for informal GSM payphone operators, especially in developing countries, is provided, as no specialised payphone equipment, apart from the SIM card 14, is required to run a payphone business.