A communication system including a plurality of terminals; and a switchboard connected to the terminals, the communication system comprising: means for causing each of the terminals to register a user identifier, one or more of possible communication methods of the terminal itself, an address of the terminal itself corresponding to the one or more of possible communication methods, and one or more of port numbers corresponding respectively to the possible communication methods in the switchboard; means for, when a voice communication between a first terminal and a second terminal is established, causing the switchboard to register a second user identifier for the second terminal as a user identifier for a communication partner of the first terminal and to register a first user identifier for the first terminal as a user identifier for a communication partner of the second terminal; means for causing the first terminal to inquire the switchboard about a possible communication method corresponding to the second user identifier, an address corresponding to the second user identifier, and a port number corresponding respectively to the possible communication method corresponding to the second user identifier, while the first terminal is holding a voice communication with the second terminal; means for causing the switchboard to search, in response to the inquiry from the first terminal, the second user identifier and to search a possible communication method corresponding to the searched second user identifier, an address corresponding to the searched second user identifier, and a port number corresponding respectively to the possible communication method corresponding to the searched second user identifier; means for causing the switchboard to transmit the searched possible communication method, the searched address, and the searched port number to the first terminal; and means for causing the first terminal to establish a communication with the second terminal, by the possible communication method received from the switchboard, using the address received from the switchboard and the port number corresponding to the communication method. means for causing each of the terminals to register a user identifier, one or more of possible communication methods of the terminal itself, an address of the terminal itself corresponding to the one or more of possible communication methods, and one or more of port numbers corresponding respectively to the possible communication methods in the switchboard; means for, when a voice communication between a first terminal and a second terminal is established, causing the switchboard to register a second user identifier for the second terminal as a user identifier for a communication partner of the first terminal and to register a first user identifier for the first terminal as a user identifier for a communication partner of the second terminal; means for causing the first terminal to inquire the switchboard about a possible communication method corresponding to the second user identifier, an address corresponding to the second user identifier, and a port number corresponding respectively to the possible communication method corresponding to the second user identifier, while the first terminal is holding a voice communication with the second terminal; means for causing the switchboard to search, in response to the inquiry from the first terminal, the second user identifier and to search a possible communication method corresponding to the searched second user identifier, an address corresponding to the searched second user identifier, and a port number corresponding respectively to the possible communication method corresponding to the searched second user identifier; means for causing the switchboard to transmit the searched possible communication method, the searched address, and the searched port number to the first terminal; and means for causing the first terminal to establish a communication with the second terminal, by the possible communication method received from the switchboard, using the address received from the switchboard and the port number corresponding to the communication method.