[Problem] To provide a voice communication system which uses a communication network, and with which a terminal device is capable of listening to both a previous call and a new call, even when another call is received during communication.[Solution] Provided is a relay device which is equipped with: a network communication unit which communicates with terminal devices over a communication network; and a control unit which controls voice communication between the terminal devices. The control unit of the relay device forms a plurality of communication sessions, and allows a plurality of the terminal devices to each of the communication sessions. The terminal devices belonging to one of the communication sessions participate in communication in the communication session. The terminal devices assigned to two or more communication sessions participate in communication in any one of the communication sessions. Voice signals received from terminal devices are transferred to other terminal devices participating in the communication session in which the aforementioned terminal devices are participating, and to terminal devices only belonging to the communication session.