PURPOSE: To perform an arbitrary specific service by performing a processing corresponding to a specific service being called from a terminal control task and to be offered.

CONSTITUTION: When a calculation service specific number is depressed first from a multifunctional telephone set 20 and a central control unit 30 receives it in a calculation service, the terminal control task 203 corresponding to a service request is generated. Next, arithmetic processing input information is inputted from the telephone set 20. A started up terminal control task 203 performs an arithmetic processing by using input information and a specific service processing function routine 204. A computed result is received by a line circuit 50 by using an output function routine 205 via a control bus 40, and is displayed visually on the telephone set 20.