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 l

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.