A method for switching connections between an IP-only phone and a soft phone to an IP gateway server is disclosed. An identical telephone number is allocated to the IP-only phone and the soft phone. When the soft phone has been moved such that a connection destination to a local-area network changes from a first connector to a second connector, a relay device is also changed from a first relay device to a second relay device. After recognizing its present position by a MAC address of the relay device to which it is currently connected, the soft phone issues a request to an IP gateway server to change an IP address to the soft phone when it is determined that the soft phone is located far away from the IP-only phone. When it is determined that the soft phone is located near the IP-only phone, the soft phone issues a request to the IP gateway server so that the IP address is changed to the IP-only phone.