PURPOSE: To secure the automatic calling to the called subscriber at the moment when the called subscriber is put under the idle state, by securing the queuing state with the busy tone being heard and with no special operation given in case the calling subscriber encounters the busy state of the called subscriber.

CONSTITUTION: When calling subscriber SUBA has dialing in the normal way to call called subscriber SUBB, the called number is detected by register control circuit REGC and through originating register OR to be then memorized in memory device MU via central controller CC. Controller CC retrieves the number group table memorized in device MU via the called number received from SUBA, and then gives the busy checking to the called subscriber retrieved. And if the busy state is decided, controller CC transfers the storage position information of the trunk which is used by SUBB for talking plus the calling subscriber storage position information to trunk scanner TSC and busy extension monitor unit BSS through buses SP BUS in order to give monitoring for the idle state of the called subscriber.