Private automatic branch exchange system

Disclosed is a PBX system connected through a communication line (61) to an ISDN public network (101), comprising a control unit connected to a PBX switching network. The control unit comprises a unit for adding a line accessing number (α) to a calling party address (#B) in an incoming call information (ii) received through the communication line so as to form an incoming information (iii) to be received by said extension terminal, and a unit for transmitting the incoming information including the line accessing number to the extension terminal in which the line accessing number is displayed, whereby the user can recognize whether the incoming call is from the ISDN public network or from another extension terminal accommodated by the PBX system.