PURPOSE: To improve handleability and to hold a call by means of a special number operation even if a general telephone set which does not have a holding function is connected by providing the holding means of a main wire call and a doorphone call in a telephone exchange and the mutual switching means of the call.

CONSTITUTION: The main device of the telephone exchange 1 is provided with main wires (telephone lines) L1 and L2, modular jack inserting terminals MJ1 and MJ2 for a telephone set Ta, terminals T1 and T2 for connecting the general telephone set on the market and terminals D1 and D2 for connection a doorphone D. The holding circuit 11, a PB signal (special number operation) reception part 12, a hooking detection part 13, a telephone set connection signal detection circuit 14 and a connection signal transmission switch 15 are provided in the telephone exchange 1. Relays r12-r15, and RL2-RL5 are required for the mutual switching operation of call holding signals and a telephone set connection signal detection operation. Since the holding circuit operates even if the telephone set is removed from a telephone plug socket during the call, the call is restored even if the telephone set is connected to the socket of the other telephone set.