Asterisk supports T.38 fax passthrough. Asterisk will not be a client, server or any form of gateway. Currently fax passthrough is only implemented in the SIP channel for strict SIP to SIP calls. If you are using chan_local or chan_agent as a proxy channel, T.38 passthrough will not work.

UDPTL is handled very much like RTP. It can be reinvited to go directly between the endpoints, without involving Asterisk in the media stream.

References:

chan_sip.c
udptl.c

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Asterisk developer's documentation :: Codename Pineapple

T38 fax passhtrough :: UDPTL