[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.