PURPOSE: To prevent a ringing tone caused by increment of a reflected wave and at the same time to prevent the occurrence of distortion, by inserting a loss of several dB into each input level only when a caller, a called subscriber and an exchange are all in the busy mode.

CONSTITUTION: When a call among three persons is requested through a switchboard 3 and by the operation of an operator, a central processor 9 connects the channels of a time division switch network 6 via a channel controller 8 and sets a trunk 7 in a 3-person calling state. The trunk 7 gives a loss of 3dB to the audio signal of time division containing the audio signals given from an incoming trunk 1, an outgoing trunk 2 and a cord trunk 5 through an attenuator. Then two audio outputs among the trunks 1, 2 and 5 are added and synthesized to be fed to another trunk.