PURPOSE: To prevent other lines from being acquired in error due to hooking of a caller by restoring the start state of a line when the line is connected to an attendant board of special number and the operator gives up the line before the caller is restored.

CONSTITUTION: It is suppose that subscribers 1, 2 accommodated in an exchange 3 make a call to the attendant board 12 of special number accommodated in a group selector 10 of an S×S exchange 7 via PCM terminal station devices 4, 6, the operator responds to the call and the line is given up by the end of call of the operator before the caller hooks on the handset. In such an operation of a special number call, a signal converter 14 in the PCM terminal station device is provided with a means for restoring the start state by the end of call of the operator. Through the hooking for several times executed by the caller after that, it is prevented that other lines 8, 9 are acquired in error via a final selector 11.