Some MGCP call agents do not properly pass those portions of Session Description Protocol (SDP) messages that advertise T.38 and NSE capabilities. As a result, gateways that are controlled by these call agents are unable to use NSEs to signal T.38 fax relay to other gateways that use NSEs. As of Cisco IOS Release 12.2(13)T, you can configure gateways to use T.38 fax relay and NSEs even though those gateways may be unable to negotiate those attributes by themselves at the time of call setup.The mgcp fax t38 gateway force command provides a way to ensure gateway-controlled T.38 fax relay between an MGCP gateway and another gateway. The other gateway in the negotiation can be an H.323, Session Initiation Protocol (SIP), or MGCP gateway. Both gateways must be configured to use NSEs to signal T.38 fax relay mode switchover. On H.323 and SIP gateways, use the fax protocol t38 nse force command to specify the use of NSEs for T.38 fax relay. On MGCP gateways, use the mgcp fax t38 gateway force command.
For those who may be interested I finally managed to get t38 fax from MGCP to SIP working.
Firstly, it seems that super g3 does not like operating at super g3 speeds across SIP, therefore I had to ensure that any super g3 fax machine operated at g3 speeds. In mgcp the command is mgcp fax-relay sg3-to-g3 and in sip it is fax-relay sg3-to-g3 under the dial peer or in global config.
Secondly my SIP SP does not support nse for the switchover from a voice call to a t38 fax call. NTE's are supported and a debug of ccsip messages shows that my provider uses a payload value of 101.
So, with this information I was able to add the following commands to my mgcp gateway:-
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