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Symptoms:  In case of MSBG , SIP signalling also gets transmit between SBC and TDM gateway for T38 REINVITE. 
If SBC is sending below format m lines in SDP then "TDM gateway" will send invalid SDP, due to which T.38 Fax will not work.
T.38 REINIVTE from SBC to TDM gateway has two m lines where for audio m line port is 0.

m=audio 0 RTP/AVP 19
m=image 61192 udptl t38


m=image 0 udptl t38
m=image 0 udptl 19

Note: wireshark on all interface (including TDM gateway local ip) can show above call flow.