PURPOSE: To dispense with a button operation and to decrease the burden of an operator by detecting a first voice part and turning a control circuit on when the B board operator of a telephone exchange responses to an incoming call to the B board in a voice.

CONSTITUTION: When the incoming call is relayed from a station line, private line trunks 10 and 11 or an extension terminal 12 through an exchange 12 to a B board 8, a buzzer 7 is sounded. When the B board server responses in the voice in putting on a headset 6 for a switchboard operator, a first part of the voice is detected 1, and a detection signal dS is sent to a control circuit 3. The control circuit 3 sends a response signal to the exchange 9, executes the connection of the receiving call and the switchboard operator, and a mutual call is executed through a circuit network 2 for telephone sets. Thus, the operator need not operate a button operating part 4, and the burden of the operator can be decreased.