PURPOSE: To not dial a central office line number or a central office line trunk number but dial only a holding response special number at the holding response time to transfer a call by providing a transfer group storage circuit for everyextension subscriber and a holding trunk temporary storage circuit for every transfer group.

CONSTITUTION: If an extension subscriber A(EXA)21 dials a holding special number after dialing '1' or hooking when the EXA21 is busy by a terminating trunk line, a central controller (CC)4 holds a terminating central office line trunk (CICT)5.At this time, the CC4 reads out a transfer group, to which the EXA21 belongs, from a transfer group storage circuit 6 for every extension subscriber. If it belongs to transfer group 0, the number of the CICT5 is stored in the area of transfer group 0 in a holding trunk temporary storage circuit 7 for every transfer group. When an extension subscriber B(EXB)22 dials the holding response special number, the CC4 reads out a transfer group, to which the EXB22 belongs, from the storage circuit 6. If it belongs to transfer group 0, said holding trunk is read from the storage circuit 7, and this trunk and the EXB22 are connected.