A call control system comprising a call controller for performing a call control by selectively using a plurality of communication networks and a plurality of terminal devices, the call controller comprising: a receiving-side terminal device current state monitoring unit operable to, when a terminal device on a transmitting side transmits a transmission request for a terminal device on a receiving side to the call controller, acquire via a plurality of communication networks from the terminal device on the receiving side, available network data representing a communication network available to the terminal device on the receiving side; an available network data holding unit operable to hold the available network data acquired by the receiving-side terminal device current state monitoring unit; and a call control unit operable to, upon receipt of a reception request from the terminal device on the receiving side, concurrently transmit call control signals together with identical call identifiers for identifying an identical call to a plurality of communication networks, respectively, estimated to be available to the terminal device on the receiving side, based upon the available network data held by the available network data holding unit, and operable to, upon receipt from each of a plurality of communication networks of each of call responses to the call control signals concurrently transmitted, determine a communication network to be used by the terminal device on the receiving side based upon call control condition information representing a condition relating to the call control and communication resource information included in the call response received in the earliest order, each of the plurality of terminal devices comprising: a communication unit operable to communicate with the exterior; and a terminal-side call control unit operable to, when the communication unit receives the call control signals concurrently transmitted through the plurality of communication networks, respectively, estimated by the call controller to be available to the terminal device on the receiving side, return to the call controller a call response including the communication resource information together with the identical call identifier through each of the plurality of communication networks estimated to be available to the terminal device on the receiving side. a receiving-side terminal device current state monitoring unit operable to, when a terminal device on a transmitting side transmits a transmission request for a terminal device on a receiving side to the call controller, acquire via a plurality of communication networks from the terminal device on the receiving side, available network data representing a communication network available to the terminal device on the receiving side; an available network data holding unit operable to hold the available network data acquired by the receiving-side terminal device current state monitoring unit; and a call control unit operable to, upon receipt of a reception request from the terminal device on the receiving side, concurrently transmit call control signals together with identical call identifiers for identifying an identical call to a plurality of communication networks, respectively, estimated to be available to the terminal device on the receiving side, based upon the available network data held by the available network data holding unit, and operable to, upon receipt from each of a plurality of communication networks of each of call responses to the call control signals concurrently transmitted, determine a communication network to be used by the terminal device on the receiving side based upon call control condition information representing a condition relating to the call control and communication resource information included in the call response received in the earliest order, each of the plurality of terminal devices comprising: a communication unit operable to communicate with the exterior; and a terminal-side call control unit operable to, when the communication unit receives the call control signals concurrently transmitted through the plurality of communication networks, respectively, estimated by the call controller to be available to the terminal device on the receiving side, return to the call controller a call response including the communication resource information together with the identical call identifier through each of the plurality of communication networks estimated to be available to the terminal device on the receiving side. a communication unit operable to communicate with the exterior; and a terminal-side call control unit operable to, when the communication unit receives the call control signals concurrently transmitted through the plurality of communication networks, respectively, estimated by the call controller to be available to the terminal device on the receiving side, return to the call controller a call response including the communication resource information together with the identical call identifier through each of the plurality of communication networks estimated to be available to the terminal device on the receiving side.