PURPOSE: To afford facility for both a calling side and a called side, by sending out a call signal and a ringing tone after discriminating them from those which are connected usually, in case when an incoming call is connected to an extension telephone set other than that of a dial number owing to an absence transfer service, an extension representation function rotary service, etc.

CONSTITUTION: In case when an incoming call is connected to an extension telephone set C other than a telephone set B of a dial number, this information is transferred to a memory device MEM through a common controller CC, and is stored so that the following incoming to the telephone set B is transferred to the telephone set C. In this state, when the extension number of the telephone set B is dialed from an extension telephone set A or a trunk attendant board ATT which has received a trunk incoming, the device CC which has received said number collates it with the store information of the circuit MM, and connects the telephone set A and a trunk COT to the telephone set C through an intraoffice trunk IOT and a frame F, and the telephone set C through the frame F, respectively.