PURPOSE: To attain the connection with a line subscriber having no reply/ restoration signal transmission function by adopting the constitution such that trunk line to trunk line connection is restored forcibly to the initial state after a prescribed measured connection time elapses.

CONSTITUTION: When an extension subscriber 1 hooks on to make transfer connection of a call of a line subscriber A to a line subscriber B, on-hook detection information is sent to a central controller 5. The controller 5 collates restoration information of the subscriber 1, hold information of a trunk line 3 stored in a storage device 7, and connection information between the subscriber 1 and a trunk line 4 to make connection command between the trunks 3, 4 and connection release command between the subscriber 1 and the trunk 4 and to start a time monitor circuit 8 thereby measuring the connection time between the trunks 3 and 4. When a prescribed connection time elapses, the connection restoration information is sent to the controller 5 and the controller 5 collates the information with the connection information stored in the device 7 to interrupt forcibly the connection between the trunks 3, 4 and to restore the state to the initial state.