PURPOSE: To eliminate the need for a backup power supply, by preventing sudden disconnection of lines at power failure, through the changeover of a main trunk to a telephone set for use in power failure automatically, after the transmission of message for a prescribed time on mains failure.

CONSTITUTION: When one of a button telephone sets 2 is communicated, then a call current detection section 4 continuously detects a call current and a power failure detection section 13 monitors a voltage of the power supply 10. If power is failed and no power is supplied to a switching circuit 15, the circuit 15 is automatically reset, a main trunk 1 during call is kept with a DC loop closing section 17 and a message representing the generation of power failure is trans- mitted from a message transmission section 18. A switching control section 14 stops the section 18 after a prescribed time and controls the switching circuit 15 to switchingly connect the main trunk 1 to a telephone set 16 for use in power failure.