PURPOSE: To control the volume of a calling sounder by switching the changeover switch of a telephone set for an analog line to a digital telephone transmitter-receiver side at the time of on-hook, and driving a sounder according to the detected result of a calling signal.

CONSTITUTION: A changeover switch 17 is connected with a telephone interface circuit (i) side in the on-hook state, a digital line side is selected by a telephone set (t), and at the time of calling from a digital telephone line, a signal control part 05 drives a sounder 10 for calling. Also, at the time of calling from an analog telephone line, an analog line calling detecting circuit 19 outputs a calling signal detection result to the signal control part 05, the signal control part 05 drives an amplifier 09 and the sounder 10 for calling, and stops the output at the time of detecting an off-hook state, and the changeover switch 17 connects the telephone set (t) with an analog telephone subscriber's line terminal 00 side. Then, at the time of detecting the on-hook, the signal control part 05 returns the changeover switch 17 to the telephone interface circuit (i) side, and the original line connecting state is obtained.