RFC 3362
T.38 fax
Fax can switch from audio to T.38 mid-call. A voice codec that looked fine will still fail a page.
- INVITE — The call starts as audio so the fax tones can be heard.
- re-INVITE — T.38 is offered when the tones are detected, if both legs allow the switch.
- T.38 — If the carrier stays on G.711 passthrough, do not force T.38 on only one leg.
One leg on T.38 and the other on passthrough is a broken fax even when the SIP ladder shows 200 OK. NSE-based fax or passthrough for secure SIP lines is called out in the CUCM 15 gateway notes dated February 2026. Read that note before you change a secure fax trunk.