PURPOSE: To enable to locate the subscriber unit at an arbitrary subscriber terminal at the end of communication, by providing the communication line monitor means with the subscriber unit.

CONSTITUTION: After the end of communication, when the line subscriber telephone SUB is hooked, on although the connection of SUB-IOT-SUBU is disconnected, since the subscriber unit SUBU is in response state, connection is changed to the busy tone transmission trunk BTT with the path of (2). When SUBU is changed for the connection from the intra-office trunk IOT to BTT, the communication current fed to SUBU is disconnected, relay LP is restored, and SUBU is restored. Further, the current fed from BTT with the capacitor C2 is interrupted, and since the communication current supply relay A is not operated, BTT discriminates SUBU in restoration and restores the connection of SUBU-BTT.