When multiple persons use the same telephone set installed in a home environment or in a work environment lacking a switchboard, the amount of the periodic invoice for the telephone service can grow considerably and uncontrollably. In this case the holder of the contract with the telephone service provider generally feels the need to reduce the periodic expense or to make the different users contribute fairly to its payment, usually according to the use that each has made thereof.

Backzround Art Means to solve the problem do exist, but they present drawbacks both in the control and in the use of the telephone set.

For instance, locking the keypad could be a means to reduce the cost of the service by limiting the use of the telephone set only to persons who have the related key, but it prevents its use in case of certain calls such as emergency ones which, additionally, are usually free.

On the other hand, disabling long-distance calling capabilities may help prevent the cost of the service from increasing, but only to a certain extent, since the introduction of a time-based local calling charge weighs heavily on the overall cost of the service.

For this purpose, the installation of a unit counter can constitute a

psychological deterrent against prolonged conversations, and could be useful to allocate the cost of the service, if each person recorded the amount of the related cost at the end of the conversation. This obviously requires the persons using the set to trust each other and to make a commitment to record the duration of each of ; u their calls.

A means similar to the unit counter is the record of calls enclosed to the invoice by the telephone service provider. However, it does not show calls of less than four units and it would still be necessary for all persons using the telephone set to recognise the listed telephone numbers and to accept responsibility for the calls made.

Disclosure of Invention An aim of the present invention therefore is to solve the problem of reducing the amount of the periodic invoice for the telephone service, limiting the number of persons authorised to use the telephone set, or to make the different users contribute fairly to its payment, generally according to the use made thereof by each.

Within the scope of the solution to this problem, a further aim of the present invention is to allow the indiscriminate use of the telephone set for emergency calls.

Yet another aim of the present invention is to allow easily to determine the amounts due by all persons authorised to use the telephone set.

The invention, such as it is characterised by the claims that follow, solves the problem of providing a device for enabling multiple persons to use a private telephone set and for charging the related calls to them, which from a general standpoint is characterised in that it comprises : - a central microprocessor control unit, including a non-volatile memory unit in bidirectional connection therewith and, in unidirectional connection therewith, a keypad for storing therein a code, a credit, emergency numbers and all other

useful information ; to the control unit being uni-directionally connected : - means for recognising a person authorised to use the telephone set ; - an alphanumeric display for showing the credit of the person authorised to use the telephone set ; and, also connected to the telephone line upstream of the telephone set : - an incoming call detector, constituted by an electronic circuit able to detect the line current sent to activate the ringer of the telephone set and to give an assent signal to the control unit ; - a line pulse detector, constituted by an electronic circuit able to detect line counting pulses and to give an assent signal to the control unit ; - a line drop device, constituted by a local tone generator and by a relay, able to switch the telephone set from the telephone line to the local tone generator, and capable of receiving assent for switching from the control unit ; - a detector of dtmf tones or dialling pulses, constituted by an electronic circuit able to detect and decode the numbers dialled on the keypad of the telephone set ; - a handset hang-up detector, constituted by an electronic circuit able to detect whether the handset is lifted or not and to communicate it to notify the control unit.

Further features and advantages of the invention shall be made more readily apparent from the detailed description that follows of a preferred embodiment illustrated purely by way of non limiting example in the accompanying drawings Description of the Drawing - Figure 1 shows a block diagram of the main components of a device according to the present invention.

Description of the Illustrative Embodiment In accordance with the present invention, a device for enabling multiple

persons to use a private telephone set and for charging the related calls to them is inserted on the telephone line exemplified by a twisted pair L upstream of a telephone set shown schematically as a block T. As indicated in the respective blocks of the diagram in Figure 1, the device according to the invention comprises a central microprocessor control unit 1, controlled by a software.

The control unit 1 includes a non volatile memory unit 2 in bidirectional connection or integrated therewith. In the memory unit are stored the access codes or passwords of the persons authorised to use the telephone set, any telephone credits they may have, emergency telephone numbers always available for dialling through the telephone set, and other useful information of the control unit 1 which, as shall become more readily apparent below, performs a supervisory function in the device according to the present invention.

To the control unit 1 is connected uni-directionally a keypad 3 to store in the memory unit 2 authorised persons'passwords, credit, emergency numbers and any other useful information as mentioned above.

To the control unit 1 are also uni-directionally connected means for recognising a person authorised to use the telephone set.

These recognition means comprise a card reader 4 able to analyse and recognise a valid electronic card and notify the control unit 1.

In the absence of a card reader, the keypad 3 can be used as a recognition means, since from the keypad 3 it is possible to modify in the memory unit 2 both authorised persons'access codes and their credits.

Normally, as shall be specified farther on, the keypad of the telephone set T is used to recognise the authorised person.

Additionally a coin machine 5, connected uni-directionally to the central control unit 1, is able to recognise valid coins and to notify the control unit 1 itself, in the eventualities specified below.

At the output of the control unit 1 is provided an alphanumeric credit display

6 whose purpose is to indicate the current credit of the person authorised to use the telephone set.

To the control unit 1 are further connected uni-directionally the following electronic circuits, in turn connected to the twisted pair L of the telephone line upstream of the set T : an incoming call detector 7, able to detect the line current sent to activate the ringer of the telephone set and to give an assent signal to the control unit upon detection of the line current ; a detector 8 for detecting line pulses, i. e. time units, able to detect the counting line pulses and to give an assent signal to the control unit ; a line drop device 9, constituted by a local tone generator 10 and by a relay 11, able to switch the telephone set T from the telephone line L to the local tone generator 10, able to receive the assent for switching from the control unit 1 ; a detector 12 for detecting dtmf tones or pulses, uni-directionally connected to the telephone line L and able to detect and decode the selections set on the keypad of the telephone set ; a handset hang-up detector 13, able to detect whether the telephone handset is lifted or not, by checking the current absorbed by the telephone set T and to notify the control unit 1.

Hereafter an explanation is provided of the operation of the device according to the invention, which, as previously mentioned, is inserted on the telephone line L upstream of the telephone set T.

The device according to the invention is able to inhibit calls by sectioning the telephone line L. When lifting the telephone handset'T, a person who is about to make a telephone call does not hear the exchange tone, but a different tone generated locally by the local tone generator 10. To have the line, an authorised person must let him/herself be recognised through one of the means described above, for instance by inserting his/her card into the card reader 4, or by typing

his/her access code on the keypad 3 connected to the control unit 1 or on the keypad of the telephone set T through the tone detector 12, when the line drop device 9 is switched into local tone generator 10.

Once the person ho,,, ils about to make a call has been recognised as an authorised person, if his/her credit present on the card or read in the memory unit 2 is judged sufficient by the control unit 1, he/she can proceed with the call.

His/her telephone credit is shown on the display 6 and it decreases during the call until it reaches zero or as the call itself is prolonged it may assume a negative value, continuing the count. In the first case, the telephone call is interrupted and the device returns to the initial mode, i. e. as a sectioned telephone (local call), and in order to place another call, the same person will need new credit from the control unit 1 or will have to insert coins into the coin machine 5. In the second case, that telephone call will go on with the continuation of the negative count and the amount reached will be regularly charged along with all calls placed subsequently.

If there is an incoming call, it is automatically recognised by the device and let through to the telephone set T allowing the user to answer and carry on the conversation until the end of the call, detected by the handset hang-up detector 13.

Incoming call recognition is performed by the telephone line call current detector 7, which activates a routine on the software of the control unit 1 that bypasses the sectioning of the telephone set T to allow it to be picked up, until the end of the conversation.

For emergency calls, the device provides an emergency code, which bypasses the sectioning of the telephone set T, allowing to dial from the keypad only the numbers previously stored in the memory unit 2.

It is evident that in its embodiment the device according to the invention can be integrated in the telephone set itself.

In practice, modifications and/or improvements are obviously possible without thereby departing from the scope of the claims that follow.