PURPOSE: To reset freely a transferred extension number set once from a moved destination to utilize effectively a device by enabling to change freely the extension number of a transfer destination from a key telephone in a system.

CONSTITUTION: Plural key telephones 301∼30n provided with dial buttons 31 and extension selecting buttons 32 are connected to the telephone interface circuit 11 of a main equipment by a local speaking circuit 20. A channel switching circuit 12 and a data control circuit 13 are connected to the circuit 11, and the circuits 12, 13 are controlled by a CPU circuit 14 connected to an RA circuit 15. The system is enabled to change freely the extension number of the transfer destination from the telephone sets 301∼30n by controlling the circuit 14 of the system. Thus, the extension number of the transfer destination set once can be reset freely from the moved destination to utilize effectively the key telephone sets.