Does anybody know if it is possible to use fax relay and fax passthrough at the same time on a MGCP gateway?
Great information from Allan, but the answer is it is one or the other with MGCP. CUCM4.2 and CUCM6 are the only versions of CUCM capable of controlling the gateways running T.38. If you aren't running one of these versions of CUCM, I would stick with fax passthrough and leave the mgcp fax t38 inhibit command in place. I would also check to make sure you have this command in place:
no ccm-manager fax protcol cisco
as well as under voice service voip this command:
fax protocol pass-through g711ulaw
I hope this helps!