PURPOSE: To realize the improvement of service to a caller by giving voice information to a multiplex response waiting call that the extension is busy.

CONSTITUTION: When a night response extension 1 is busy and a trunk line incoming call is given sequentially to trunk line trunks (TRK) 3, 4, a central controller (CC) 7 detects the presence of incoming to the TRK3 at first, references a data in a storage device (M) 8 to discriminate whether the night response extension is busy or not. Since the extension 1 is busy, the CC7 gives a command to a switch (SW) 9 to set a link 2, and the extension busy state is informed in voice to a caller of the trunk line incoming call from an announcement device (ANS) 5 via the link 2 and the TRK3. Then the CC7 writes the trunk line incoming call to the TRK3 into an incoming response waiting area (Q) 81 so as to apply restoring supervision of the night response extension 1. When the transition of the night response extension 1 to the idle state is recognized, the CC7 gives a command to the SW9 to set a link 3 and a call signal and a call tone are transmitted respectively to the night response extension 1 and the TRK3. When the night response extension 1 responds to it, the CC7 detects it to erase the 1st response wait call from the Q81.