1.5 Administrator's Guide

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  7. Legacy PBX Remote extension gateway

Legacy PBX Remote extension gateway


A company has a head-office with 4 FXO trunks to the telco and 10 analogue extensions on a legacy PBX. The legacy PBX has a number of unused extension and trunk ports available. The company is opening a second branch, geographically separated from the first, and desires a stand-alone PBX at the branch, with its own BRI telco interfaces, but the ability for head-office and branch to phone one another using extension numbers across a SIP trunk. There will be four extensions required at the branch, with potential for some growth.


Install a Com.X at both sites. For the branch, the Com.X serves as a PBX, with the BRI outbound routes routing across the BRI interface.

In the head-office, place the Com.X behind the legacy PBX, connecting its FXO ports to free FXS ports on the legacy PBX and configuring the SIP trunk to the branch.


In this configuration, the branch extensions look like additional extensions on the legacy PBX, but the Com.X gateways these across the SIP trunk to the branch.

Calls from the branch to head-office are routed across the SIP trunk, prepended by caller ID, and the Com.X at the head-office then routes based on the branch CID to the appropriate FXO port, in effect using the correct legacy PBX extension for activity from the associated branch extension.


  • At the branch, create a FlexPath with a Match rule that matches head-office extensions with an outbound route as destination and the following transform: CD.

  • At the head-office, add FlexPaths with Match rules that match the DIDs transformed by the branch and routes them based on the received CID number in the DIDs to the correct FXO port.

  • At the head office, assign DIDs to the Com.X FXO ports and add a FlexPath that pre-pends this DID to the dialed number before routing to the branch across the SIP trunk.

  • At the branch, add FlexPaths with Match rules that routes to the appropriate extensions based on the DIDs received from the head-office.