VOIP is a sticky-wicket when you start talking about fax.
Invented for voice communications, it literally, it means “voice over internet protocol”. A few lost packets here and there won’t make much difference when you are talking to Aunt Sally. She will still understand what you are saying.
When you start talking about fax transmissions, however, a lost packet here and there spell disaster to your fax session. A preamble precedes each frame in the communication stream that indicates what type of frame it is. Following that is is a sequence of bits defining the purpose of the frame. Finally, a final bit sequence indicating, among other things, whether there are additional frames to follow. Fax devices misinterpret the frame sequences when lost packets occur. Mayhem results.
This is, of course, an over-simplification. The VOIP vendors have historically hidden this fact from prospects. They will assure you “Oh, yes VOIP can handle fax very well”. Don’t believe it! This issue led to a class action lawsuit some years ago with a major VOIP provider.
With VOIP phone service, you will likely need to lower your baud rates to 9600 and most likely even further to 4800 bps in order to achieve any reliability. If you are stuck with your VOIP phone provider, your best option is to use an internet fax server like FaxAge.com or SRFax.com. Snappy Fax interfaces nicely with both.