FAX Support - Why is faxing over sip not supported?

One advantage of SIP is compression. You can combine data, voice and video, broken into packets, over an SIP network in compressed form — to save bandwidth and, usually, maintain acceptable quality. With a VoIP phone call, dropping packets is annoying, but the connection survives.

Faxes, however, cannot be compressed, and it actually takes more bandwidth to send a fax over SIP than a regular phone call. Worse, even a small amount of packet loss along the way can cause the entire fax to fail. The longer the fax, the less likely it is to go through. And that’s only one of many problems fax has with SIP.

That is why we do not support faxing over sip and recommend placing critical fax applications on plain old telephone lines (also known as a POTS line), since faxing is known to be problematic on SIP networks.

Why the Benefits of SIP Don’t Accrue to Fax?
Compression is a key benefit of SIP, letting you converge and compress voice and data on the same pipe and save on bandwidth. But you can’t compress a fax, so there’s no savings to be had.

Why Fax Can Fail over VoIP/SIP

Packet delay, jitter and loss: While mostly reliable, VoIP (also known as SIP trunking) can delay or drop a packet here and there, by design. But even a 1% packet loss can kill an entire fax
transmission.
Protocol incompatibility: For example, many VoIP services use the G.729 protocol to compress voice calls; since faxes cannot be compressed, they require the standard-rate G.711 protocol. The brief gaps in fax tones that occur as the system tries to negotiate between the two protocols can cause the fax to fail.

Even Fax-over-IP protocols have inefficiency and reliability problems

G .711 - Considering that the G.711 protocol is actually designed to digitize voice, converts fax tones into a digital signal at 64kbps. Packetizing the G.711 signal adds SIP overhead totaling 88Kbps, which is 38% more bandwidth than a standard voice call and 175% more than a VoIP call compressed to 32Kbps
T.38 - The newer T.38 protocol was intended to transmit faxes directly over SIP (FoIP), so the fax doesn’t need to be converted to an audio stream first. But T.38 must be on both ends of a
network to work, and many service providers never implemented the protocol. If the fax has to traverse networks that do not support T.38, it will need to be transcoded, which can ad latency, increase cost, and may cause the call to disconnect