Yes, making fax machines work over VoIP was important in the early days. The G.711 codec was designed to support endpoints that needed a full 64kbps datapath.
I've had much better success having fax calls work over VoIP then I have getting data modem calls to work. My best guess as to why is that fax calls have much less back and forth, and as a result, they are less latency-sensitive. Fax calls also usually negotiate at 14.4kbps, whereas data modem calls often try to train at faster rates.