PURPOSE: To improve the service by providing a means storing the accommodated position of a request extension to a trunk line and identifying automatically the response of the trunk line when the operator of a private branch of automatic exchange acquires a main wire trunk to relieve the load of the operator and request extension.

CONSTITUTION: An extension telephone set 1 and an attendant board 2 are accommodated to the input side of a channel switch 3 of the private branch of exchange, and the main wire trunk 4, an extension two-way trunk 5 and an outgoing register 6 are accommodated to the output side of the switch 3. Further, a central controller 7 identifying the response of a main wire subscriber is connected to the switch 3 and a storage device 8 is connected to the controller 7. When the operator acquires the main wire trunk 4, the storage device 8 stores the accommodated position of the requested extension to the trunk 4. Even when the operator and the requested extension are restored before the response of the main wire subscriber, the calling of the main wire subscriber is continued and when the main wire subscriber responds thereto, the requested extension is called back so as to relieve the load of the operator and the requested extension.