PURPOSE: To transmit an interruption tone to a fixed subscriber only and further to prevent leakage of a talk without interrupting subscribers in a talk during transmission of the interruption tone, by using a mixer trunk with fixed functions.

CONSTITUTION: When an interruption request from subscriber A to subscriber B is generated while subscribers B and C are in a talk, switching controller 2 refers to data in memory 8 to discriminate the business of B. Controller 2 connects A by acquiring sound source trunk 3-0 to send an incoming enabling tone to A and further acquires mixer trunk 5 to connect B to terminal X, C to terminal Y and sound source trunk 3-1 to terminal Z. Consequently, the channel between B and C is secured and the interruption tone from trunk 3-1 is transmitted to only B. On detection of the hooking of B,the channel between A and B is formed and holding tone is sent from trunk 3-1 to C.