Faxing in the analog world doesn't easily convert analog to digital and vice versa as a voice call does it uses a MOdulateDEModulate process best described at:-
https://www.itu.int/rec/T-REC-T.30-200509-I/en
If you understand Modems then you will see why any out of order packets in the stream will break the signalling as they rely on very strict timing of the packet strea. Now ECM is available to correct certain runtime errors but when losses creep past a couple of percent or 20ms of non-sequitor noise at the wrong place then the call will fail on a fax but be hardly noticable for a human ear which has a much better DSP behind it,
Hence T.38 which switches to HDLC coding best described at:-
https://www.itu.int/rec/T-REC-T.38-201009-I/en
Which effectively provided mechanisms for out of order packet processing by buffering and general network practices instead of strict analog G711/T.30 this is why you can T.38 fax over compressed codecs.
For a discussion of why fax is hard over IP start and to edumicate the doubters of these problems, Steve Underwood the father of spandsp which is used by Asterisk/FreePBX and many commercial fax machines, offers on Steve's site:-
http://www.soft-switch.org/foip.html
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