Fax over IP (FoIP) is finally being standardised now and most of the mainstream gateways (Quintum, Cisco etc.) can send and receive faxes.
There are several protocols for doing this, the most widely adopted is T.38, this handles most 9.6kbs and 14.4kbs fax machines, although there have been many documented issues with gateways working with fax/modem cards in PC’s.
Unfortunately it all comes down to inter-operability testing. I have set up many companies, including Fax broadcast agencies, using mostly Quintum’s successfully.
If there is something specific you are trying to acheive please feel free to contact me at: firstname.lastname@example.org
Hope this helps