A switching center 30 comprises a memory part M2, a memory part M3, and a memory part M4. The memory part M2 stores a terminating terminal number corresponding to and registered by each subscriber. The memory part M3 stores idle/busy information corresponding to each terminal. The memory part M4 stores idle/busy information corresponding to each subscriber. When a subscriber 11.sub.1 makes a call at a terminal 12.sub.2, the switching center 30 determines whether the idle/busy information for the subscriber 11.sub.1 stored in the memory part M4 is idle or busy. When the idle/memory information represents idle, the switching center 30 refers to the idle/busy information for the terminal 12.sub.2 stored in the memory part M3. When this idle/busy information represents idle, the switching center 30 reads out the terminal number of a terminating terminal of a terminating subscriber stored in the terminating terminal memory part M2 and performs an outgoing call connection process. At this point, the switching center 30 also sets the idle/busy information for the subscriber 11.sub.1 and the terminal 12.sub.2 to busy state. When the subscriber 11.sub.1 makes a call, while the terminating terminal 12.sub.1 is performing an incoming call connection process for the subscriber 11.sub.1, since the idle/busy information for the subscriber 11.sub.1 has been set to the busy state, even if the terminal 12.sub.2 is idle, the switching center 30 does not perform an outgoing call connection process for the subscriber 11.sub.1 at the terminal 12.sub.2.