When a trouble is developed in an IP exchange (TM) 2 or a circuit (network) 5, a backup exchange 6 (backup TM) is switched over from a waiting system to an operating system, and all IP terminals Eqi inside lodgments A, B, . . . N also log out from the IP exchange (TM) 2, and transmit the IP addresses to the backup exchanges 6 (backup TM) inside own lodgment, respectively, and log in there with a result that the extension telephone calls inside own lodgment is made possible by a control of the backup exchange 6 (backup TM), and therefore, even when a fault occurs in the IP exchange or the circuit, the IP terminals inside the lodgment are backed up and the actuations of the IP terminals are continuously operated, and for example, the extension phone calls inside the lodgment can be realized.