The IP-PBX 1 includes a multimedia gateway controller 2 in which a call control data master table stores call control data for all terminals falling under the control of the IP-PBX and protocol handlers which are installed on a system bus 101 or expansion system bus 102 in a sufficient number for serving the number of circuits accommodated by the IP-PBX. On each of the protocol handlers, a microprocessor performs processing tasks corresponding to a load shared on it in interfacing with a given number of terminals, using a call control data slave table in its memory, call control protocol conversion function, fault-supervisory-processing function, call control protocol retransmission function, call control protocol transfer function, and terminal supervisory function.