A relaying device comprising: a network communication unit connected to a communication network and adapted to communicate with a plurality of terminal devices over the communication network; and a control unit adapted to control voice communication between the plurality of terminal devices by transferring a voice signal received from a terminal device via the network communication unit to a terminal device of a communication partner, wherein the control unit is further adapted to: a) cause a terminal device belonging to a plurality of communication sessions to only hear in a communication session of any one of the communication sessions to which it belongs; b) cause a terminal device participating in a communication session with another terminal device to hear and speak in the communication session in which it participates; and c) wherein in a case in which a first terminal device belongs to the plurality of communication sessions and is caused by the control unit to participate in a communication session determined to have highest priority, if the communication session with highest priority in which the first terminal device is participating is cancelled and if a communication session with lower priority continues, the control unit is further adapted to cause the first terminal device to participate in the communication session with lower priority. a network communication unit connected to a communication network and adapted to communicate with a plurality of terminal devices over the communication network; and a control unit adapted to control voice communication between the plurality of terminal devices by transferring a voice signal received from a terminal device via the network communication unit to a terminal device of a communication partner, wherein the control unit is further adapted to: a) cause a terminal device belonging to a plurality of communication sessions to only hear in a communication session of any one of the communication sessions to which it belongs; b) cause a terminal device participating in a communication session with another terminal device to hear and speak in the communication session in which it participates; and c) wherein in a case in which a first terminal device belongs to the plurality of communication sessions and is caused by the control unit to participate in a communication session determined to have highest priority, if the communication session with highest priority in which the first terminal device is participating is cancelled and if a communication session with lower priority continues, the control unit is further adapted to cause the first terminal device to participate in the communication session with lower priority. a) cause a terminal device belonging to a plurality of communication sessions to only hear in a communication session of any one of the communication sessions to which it belongs; b) cause a terminal device participating in a communication session with another terminal device to hear and speak in the communication session in which it participates; and c) wherein in a case in which a first terminal device belongs to the plurality of communication sessions and is caused by the control unit to participate in a communication session determined to have highest priority, if the communication session with highest priority in which the first terminal device is participating is cancelled and if a communication session with lower priority continues, the control unit is further adapted to cause the first terminal device to participate in the communication session with lower priority.