Distributed transit private branch exchange

A plurality of M PBXs are coupled to a plurality of N nodes forming a Distributed Transit Private Branch Exchange (PBX). The plurality of N nodes are located remotely from each other and form a wide area network. Each node is coupled to predetermined ones of the other nodes via separate links so that each node is able to communicate with all other nodes. One or more D-channel Server Modules (DSMs) are located in predetermined nodes of the plurality of N nodes. Each DSM is associated with a plurality of D channels which are each coupled to a separate interface device in the same node or different node by means of a link for receiving and transmitting signaling data to one of an associated predetermined PBX of the plurality of PBXs. Each DSM is arranged to analyze the call signaling message received via a D channel and, based on routing information, either route the call back to the same PBX or a different PBX attached to the same DSM when the calling and called subscribers are located in the PBXs attached to the same DSM, or route the call to a remote DSM when the call is routed between remote PBXs that are not coupled to the same DSM. The remote DSM, in turn, routes the call to the PBX attached to that DSM. Therefore, each DSM only controls calls addressed to a DSM associated with that DSM, and when a call is not addressed to a PBX associated with that DSM, the DSM passes any call control to a destination DSM associated with a PBX to which the call is addressed.