Route any calls to your fax phone numbers across the public network through the analog cards. Or setup a fax server that will receive the fax, convert it to a file, and transfer the file to another fax server that would play the file to the distant fax machine. (Fax store and forward.)
You can’t do fax over VOIP. Fax machines typically send information at 28.8kbps or 33.6kbps. The audio codecs used by VOIP sample too slowly.
You may (MAY!) possibly get some success if you can limit the fax machines to no more than 9600bps or MAYBE 14.4kbps, but 14.4 would be pushing it. In those cases, you need to use a fat protocol like G.711 u-law or A-law.
Most fax machines have no method to limit their transmit speed. They’re usually interested in connecting at the fastest speed possible. Even if you had the ability to regulate the fax machines transmit speed, you’d probably still have a great deal of trouble.
This is not something that will ever be fixed in VOIP. If you have network connectivity necessary to establish an end-to-end VOIP call, it’s assumed that you have the ability to move a data file as well, and you should probably be moving the information by email or some similar method.
Click here for more information: soft-switch.org/foip.html