PURPOSE: To connect a call-back from a police station or the like to a proper extension telephone set raising an alarm by providing an extension number storing means for the extension telephone set raising the alarm.

CONSTITUTION: While performing access to data in a RAM 9, according to an exchange program stored in a ROM 8, a CPU 6 of an arrangement for interchanging 1 controls a speech path switch or various device parts and realizes an exchange funciton in a normal case, destination monitor funciton in the case of an emergency call, special number processing funciton and special transfer destination change funciton. When an extension telephone set A makes the emergency call, a destination monitor part 15 stores the number A in an extension number area 12 of the RAM 9. When another extension telephone set B responds to the call back for inquiring additional information, the telephone set B turns on a special number for special transfer. A special transfer part 17 of the exchanger 1 calls the extension telephone set A from the extension number area 12 and connects a line with the telephone set of the police station.