1.5 Administrator's Guide

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Legacy PBX feature set extension

Legacy PBX feature set extension


A company has a significant investment in a legacy analogue PBX. This investment includes some features, e.g. voicemail, routing, groups, feature codes and IVRs that the company would like to preserve.

They would how-ever like to extend the legacy PBX with new SIP extensions and additional features, including call recording, least cost routing and additional SIP-based features for the new SIP phones, and possibly enable the PBX to support a remote office as described in section The existing PBX cannot be extended, or the cost associated with such extension is prohibitive.

As a result, the company want existing analogue phones to be served dial-tone directly from the existing PBX (preserving the existing feature set) and the new features and LCR to be available to new SIP extensions. Call recording should be available on all extensions, and it should be possible to block extensions from dialing.


Install a Com.X solution with sufficient analogue FXS and FXO ports for the company. The Com.X solution provides SIP extensions, LCR and the other features mentioned to the business.

Connect the analogue phones to the Com.X solution’s FXS ports and the Com.X solution’s FXO ports to the existing PBX’s FXS ports.

For each FXO port, assign a DID matching the legacy PBX’s extension. Create an extension with the same extension number for each of the legacy PBX’s extensions.

For each of these FXS ports, create an outbound route with dial pattern ‘!’  and a FlexPath with the extension FXS port as the source, and destination the associated outbound route. Enable immediate dialing on the extension port 

Using this configuration, when an analogue phone goes off-hook, the legacy PBX would be providing dial-tone directly. If the legacy PBX’s extension rings, the associated Com.X extension also rings. This enables calls to be recorded on the legacy analogue extensions. Though dialed numbers cannot be recorded in CDR records for the legacy extensions (since the number is not collected by the Com.X solution but rather passed straight through to the legacy PBX), the duration of calls originating from legacy extensions can be recorded. Extensions can also be blocked. Call recording would be fully featured for the new SIP extensions.