PURPOSE: To improve compatibility of the public telephone set, by using an audible signal as the service forced disconnection forecast tone of the single charging system or the coin supply pressing tone according to the condition of the switch.

CONSTITUTION: If the public telephone set is only for local service and is the single charging system, connectors CN3 and CN5 are short-circuited between 1-2, and timer circuit TMC started by service start transmits the audible signal, and this signal becomes the forcasting tone for forced disconnection. Next, if the public telephone set is only for local service and is the multi-charging system, connector CN5 is short-circuited between 1-2, and connector CN3 is short-circuited between 2-3, and the audible tone of timer circuit TMC becomes the coin supply pressing tone. Next, if the public telephone set permits toll call and is the multi-charging system, connectors CN3 and CN5 are short-circuited between 2-3, and the operation of circuit TMC stops, and after that, the audible signal is not transmitted.