PURPOSE: To improve the reliability of measures to a fault by remedying the fault without disconnecting a call even if the fault occurs to a line accommodation device during a communication.

CONSTITUTION: If the fault occurs to the terminal line accommodation device 501 of an operation system during the communication between terminals 600 and 700, the connection destination of the line of the terminal 600 is switched from the terminal line accommodation device 501 of the operation system to the terminal line accommodation device 502 of a stand-by system. Further, the connection destination of a time-division switch 508 is switched from the terminal line accommodation device 501 of the operation system to the terminal line accommodation device 502 of the stand-by system. Consequently, even if the fault occurs during the communication, the call is never interrupted.