PURPOSE: To relieve the load of a central processing unit as much as possible after detection of a call by writing in advance an address of a channel memory to which specific information to be connected at first to a subscriber is stored to a control memory.

CONSTITUTION: A read address of each sound source is written in advance to a time slot position on a control memory 6' of a subscriber to be transmitted. When the subscriber A hooks off for call, since the memory 6' reads already a channel memory 3, a dial tone DT is listened to immediately. When a subscriber B hooks off, the operation to start a call of other extension is executed soon and a call tone is listened to. When the subscriber C hooks off, a service tone SDT is listened to immediately. Each tone above is started regardless that a central processing unit (not shown in figure) does not give a command of write to the control memory because of exchange operation, the load of the central processing unit is relieved.