PURPOSE: To prevent disabled dialing by sending DP, PB signals to an exchange and switching automatically the dial signal into the DP, PB signals when a call tone from an exchange is not stopped to send the dial signal in matching with the DP, PB lines.

CONSTITUTION: A telephone line connecting to a key telephone set TE1 is a PB line PBL. When a DP, PB signal changeover switch 6 is thrown to the position of a DP signal of a normally closed contact 6b, the telephone set TE1 is in the DP mode. When a dial key 5 is operated in the DP mode, a call tone is not stopped even after lapse of a prescribed time by an inter-digit timing detection circuit 4b due to mismatching of the dial signal. As a result. AND circuits 7b, 7c are active and a DP, PB switching signal is sent to a CPU 9, then the CPU 9 selects a DP, PB signal automatic switching circuit 8 from the DP mode into the PB mode. Thus, a dialer section 4 sends a PB signal by the succeeding operation of the dial key 5 and a claim of disabled dialing is prevented.