Abstract: A communication system includes a plurality of communication devices, and a server. The communication devices share a phone number and respectively have an identification data. The server stores a state data corresponding to the phone number. The state data includes an allowable communication state corresponding to each of the identification data. The server allows one of the communication devices to communicate through the server according to the allowable communication state and disallows the other communication devices to communicate through the server. The allowed communication device transmits a command to the server to change the allowable communication state of the state data according to an operation of a user, that one of the disallowed communication devices is allowed to communicate through server instead, and the originally allowed communication device is disallowed to communicate instead.