[mattrp]

Fax has an ip protocol called t.38 however it is inconsistently implemented. Most services still rely on g.711 - essentially uncompressed voice - for transmission of fax tones. However there is no guarantee the end to end call will be g.711 even if specified during call setup. There is an app called eFax - unfortunately the service is not free but I as a subscriber routinely fax over the phone when it’s the only way to communicate (like with a doc office).

[tjohns]

While G.711 is uncompressed (it’s the same codec used within the PTSN backhauls), it’s still not great for fax because the fax protocol doesn’t tolerate the jitter introduced by VoIP. (Reliable latency is the one big benefit to switched networks like the PTSN.)

As I understand it (and I could be wrong), T.38 solves this by emulating an independent fax modem on each side of the analog connection, and only sending the image data over IP as opposed to sending audio.

[mattrp]

You’re right g.711 is not ideal either. Jitter is a killer. T.38 is a solution when it’s supported on both sides and it is often not. The best solution is email but apparently every title office and pediatrician in the land cannot be bothered! ;) Although thankfully many have moved on.