PURPOSE: To decide a ring-off state with no mistake by deciding a ring-off or hooking operation in a processing cycle later by the set value of a counter than a processing cycle where the final opening or closing state of a channel.

CONSTITUTION: A counter is set when a busy channel is opened and then advanced in the negative or positive direction in each processing cycle. When the channel is closed or opened again before the counter value reaches a set level, the counter is reset each time and advanced at every processing cycle. Then the open or close state of the channel is detected for decision of a ring-off/ hooking operation when the value of the counter reaches a set level. Thus, ring-off state is correctly decided even though a pseudo pulse is produced by the vibration of a hook switch in a ring-off mode.