Protocols and standards
SIP
Acronym: SIP
SIP is the signalling protocol of voice over IP: it sets up, modifies and ends calls, without carrying the sound itself.
It plays the role that dialling and ringing played on the classic network. Both parties exchange text messages to find each other, then negotiate the call's parameters, including the codec. The sound then travels in a separate stream. SIP is now the common language of IP PBXs, broadband routers and operators.
Why it matters for fax
SIP is how two pieces of equipment agree, mid-call, to drop voice in favour of a transport dedicated to fax. If any link in the chain cannot renegotiate the call this way, the fax is still treated as voice, with the risk of failure that implies.