AnswerIf the T.38 negotiation fails, GFI FaxMaker (using Dialogic Brooktrout SR140 or TE-Systems XCAPI) can be configured to use G.711 fax-pass -through instead.
G.711 fax-pass-through is where the T.30 fax call is carried in a VoIP call encoded audio format. This is sensitive to network packet loss, jitter and clock synchronization. When using voice high-compression encoding techniques such as, but not limited to, G.729, some fax tonal signals may not get correctly transported across the packet network.
The reliability of fax over IP without using T.38 is very inconsistent and is not recommended without heavy testing - especially if the call is forwarded over an internet service provider and not terminated locally. Varied results can often be found depending on network load and the terminating fax device.
More information on T.38 can be found at: