Abstract: There is provided a communication system or the like including an IP phone network, a circuit switched network, and a shared terminal capable of connecting any one of the IP phone network and the circuit switched network, wherein, when the position of the shared terminal is registered or when a call is made from the shared terminal, the subscriber management server of the IP phone network and the subscriber management server of the circuit switched network give and receive the subscriber information about the shared terminal to share the subscriber information, and a call control server of the IP phone network and a call control server of the circuit switched network mediate the call from the shared terminal with the use of the subscriber information.