Not so commonly discussed area of the Voice Communication world is the T.38 Fax Relay between multiple signalling protocols. I was tasked to complete a Fax Server install into an existing Cisco UC environment. The Fax Server communicated to the CUCM via the SIP Protocol and the existing UC infrastructure inclusive of VG224 and a 2811 ISR was controlled via the MGCP protocol.

First thing first, gain connectivity between the CUCM and the Fax Server to ensure SIP is correctly working. Make a call to the fax server and you should receive the fax signalling sounds.. done.

Now lets work on the 2811 Voice Gateway. This is an ISDN Gateway controlled via the MGCP Protocol through CUCM. However we don’t need to log into CUCM to allow the T.38 Fax Relay protocol. Enable T.38 Fax Relay Globally under the Voice Services VOIP Menu.

(Config)# voice service voip
(Config-voip)# fax protocol t38 ls-redundancy 2 hs-redundanyc 0 fallback passthru g711alaw

In the above exampled I chose to fallback to Passthru method, as this is the existing method on the Voice Gateway.

Now lets enable the T.38 Fax Relay Protocol under MGCP.

(Config)# mgcp default-package fxr-package
(Config)# no fax t38 inhibit

Now lets restart the MGCP Application. *Depending your MGCP Configuration, this may disrupt existing calls.

(Config)# no mgcp
(Config)# mgcp

Faxing inbound/outbound the UC network should now be working as normal.

The VG224 configuration is exactly the same as the 2811 Voice Gateway. As organisations still have a need to fax internally, the VG224 needs the above configuration mended as well.

Below is the summary of configuration changes made.

(Config)# voice service voip
(Config-voip)# fax protocol t38 ls-redundancy 2 hs-redundanyc 0 fallback passthru g711alaw
(Config)# mgcp default-package fxr-package
(Config)# no fax t38 inhibit
(Config)# no mgcp
(Config)# mgcp