To improve convenience of use by signaling through the line wire if there is another line wire that enables incoming signaling, when an incoming call comes to an ISDN line, while it is temporarily picked up and performing an incoming call rejection notification to the ISDN line if there is no other line wire.

For instance, an ISDN line #1 is divided into two line wires (1) and (2), and when a digital telephone system 3 side depresses a line wire key when each speech channel is in a free state, the line wire (1) is picked up temporarily. When an incoming call arrives at the line #1 in the state, the incoming call is allocated to the line wire (2), and a line wire lamp of the system 3 is performed incoming call display. Also, when an incoming call arrives at the line #1 when the line wire (2) is picked up temporarily and when the wire (1) is in a free state at that time, the incoming call is allocated to the wire (1) and the line wire lamp of the system 3 incoming call display is made. Further, when an incoming call arrives at the line #1 when the wire (2) is picked up temporarily and when the wire (1) is in use because of speech communication, etc., at that time, incoming call rejection notification is performed to the side of the line #1.