A first call control server transmits line information of a line to be used by a terminal device calls of which are controlled by the first call control server to a second call control server when congestion is detected. The first call control server receives the line information to be used in call control from the second call control server, and establishes a connection with the terminal device using the received line information. The second call control server receives the line information from the first call control server, and receives a call control request for the terminal device, the call control request being addressed to the first call control server. The second call control server responds to the first call control server with the received line information, when the call control request is received.