PURPOSE: To expand service by connecting automatically an unringing incoming call having an audible specific signal between a sender and an extension subscriber.

CONSTITUTION: When an incoming call exists from a high-order station, an incoming trunk ICT-RC1 and a bidirectional trunk BWT9' are connected. When a start signal receiving circuit of the BWT9' receives an audible specific AC signal, the required switching is attained in the BWT9' and the identification of busy state of an extension subscriber SUB16 to receive the incoming call is attained. As a result, when the subscriber is busy, the received signal is returned and when the subscriber is not busy, a switching circuit 17 is started so as to connect the SUB16 an the BWT9' via the circuit 17. In this case, the circuit 17 transmits busy information to a private branch of exchange, causing a general call not to be connected. Thus, the automatic connection of an automatic incoming device such as mini-fax or the like and bidirectional call through an attendant board are attained without using any special exclusive telephone set.