This ensures the least amount of variables in a specific network setup.

In addition, users should ensure that an “Error Correction Mode” or “ECM Mode” is disabled on their fax machines as this compounds problems and distortion regarding the compression, decompression, and transmission of data communications through a VoIP provider.

Also the “Baud Rate” should be lowered to no more than “9600″. When faxing the fax should be set to “Standard” instead of “Photo or Fine.”

In my experience faxing via voip is simply hit or miss. The problem exists when the 2 connecting fax machines, or in some cases fax modems cannot maintain a consistent handshake. The handshake occurs many times per second and is the method for which each device ensues that the other device is still listening. VOIP communications does not provide a consitently CLEAN connection, therfore during the handshaking noise may be introduced into the call causing either device to assume that the other has hung up. This noise come from the compressin being done to the data stream. Most providers operate on the codec, G.729 which uses lossy compression, which reduces the amount of bandwidth required, but can impact the call quality. Hence, a dropped call when faxing.
One method of attempting to fix this problem relies on slowing down the handshake in order allow for bad packets to simply fall off the call, and not interfere with the handshake in addition to changing the codec being used to compress the phone call to the G.711, employing no compression schemes, which requires more bandwidth but no call loss.

I have had limited sucess with vonage when faxing and no sucess with 3 other providers including sunrocket, lingo and voip.com. Your mileage may vary, i would suggest asking the voip carrier if they support a guaranteed fax solution.