https://www.3cx.com/docs/sip-trunk-faxing/#:~:text=38%20is%20not%20supported%2C%203CX,3CX%20needs%20to%20be%20restarted.
By default The 3CX Fax server uses T.38.
When an INVITE is routed to the Fax server it will initially answer with a 200 OK containing Audio codecs. Moments after this however...