PURPOSE: To prevent the effect of a fault by allotting fixedly a memory to each call (in response to subscriber or trunk) to fix a memory area during the event processing of a specific call and using forcibly only said memory area.

CONSTITUTION: When a subscriber 2 gives a call, the subscriber data address, the head address of a work area and the final address of said work area are obtained from a conversion table on the bases of the subscriber store position l. The head and final addresses are set at a base register 41 and a barricade register 42 respectively. At the same time, a central controller 4 checks that an address is set between the base and the barricade every time the data is stored in the memory and then produces an interruption when no address exists between the base and the barricade. Under such conditions, a calling analysis is carried out with reference to the subscriber data. Then a PB telephone set is connected to a PBR. Meanwhile it is assured that an access is possible only to the area allotted to the subscriber in terms of operations of the software.