PURPOSE: To attain economization by intermittently sending a signal wave of a single frequency corresponding to a DP signal generated by dialing, by transmitting it to a main subscriber's unit with a carrier and then by sending it to a main wire as the DP signal by the main subscriber's unit.

CONSTITUTION: When a calling subscriber lifts a handset, hook switch H is closed and signal generator S, carrier oscillator C and transmitter SEN are supplied with electric power to start operating. Relay A, on the other hand, is put into operation via hook switch H and dial contact Di to open its contact (a). As a result, no signal wave arrives at modulator MOD from signal generator S and no audio input reaches from a transmitter as well, so that the carrier generated by carrier oscillator C will be sent from transmitter SEN as it is without being modulated.