Fax and VoIP
To make a long story short, your analog fax was designed to work on a Public Switch Telephone Network (PSTN) line and not a digital network connection. When analog fax protocols were developed, a single point to point path between the sending and receiving fax devices, without loss of information, was assumed. VoIP on the other hand was developed to carry the human voice, in digital packets, over a network that assumed the possibility of multiple paths between points as well as a tolerance for the loss and recovery of information. Simply put, neither Analog Fax technologies nor Voice over IP technologies were developed to be compatible with the other. The fact that a fax may from time to time seem to work over a VoIP line, in no way implies that it will continue to work on that line with any reliability, and neither your fax hardware manufacturer or your VoIP provider will support this configuration.
For best performance an analog fax device should always be used with a standard PSTN line.
Some providers of VoIP services may also provide Fax over IP (FoIP) services which unlike VoIP has been developed with fax data transmission in mind but is not without it's problems.
For further reading you may want to look at the following resources:
Faxing over IP Networks http://www.soft-switch.org/foip.html
and
