Asterisk is not good at handling FAX and voice on the same analogue line. It takes a good few seconds for Asterisk to recognise FAX tones and, of course, it tries to do this on every inbound call. I am also pretty sure that there is a bug in our shared analogue/FAX handling within SAIL, which doesn't help. Analogue FAX is very rare in our customer base because most of them choose to send and receive fax digitally over their ISDN lines. Those that don't, tend to have dedicated FAX circuits.
Let us know what you decide and how you get on. If you run into trouble, just yell and we'll try to help.
Kind Regards
S