http://www.essentialfax.com/manual/essentialfax_manual.html
http://www.essentialfax.com/fax-software/technical/faxing-over-voip.html
http://askbobrankin.com/fax_over_voip.html
hylafax free fax server
http://hylafax.sourceforge.net/howto/intro.php#ss1.2
Using the hylafax server via web site
http://www.avantfax.com/