I can't guarantee that it will work for you, but I have no problems at all sending or receiving faxes on my Vonage voice or fax line. I use MS fax service on Windows 2000 and Windows XP with USRobotics/3Com external and internal fax modems.
In fact I tested extensively using my voice line before committing myself to signing up for a Vonage fax line and canceling my eFax account. I have had no problems sending and receiving multi-page (10-12 pages) business documents and graphic image faxes such as maps or equipment diagrams using either my Vonage voice line or my Vonage fax line. This included some extensive testing sending from one Vonage line to the other Vonage line. I know the Vonage to Vonage tests actually went out over the internet instead of being looped back internally because my voice line and fax line are on separate ATAs* (although I did also run tests using both my VT2442 and RTP300 configured to handle both lines to send and receive faxes on the voice line and fax line at the same time using the same ATA).
Several years ago I tried to do the same thing (using the same equipment) and it failed miserably. I can only assume that Vonage has improved their fax handling capability through firmware upgrades (both of my ATAs downloaded newer firmware as soon as I put them on-line after signing up for Vonage service again recently) and possibly server infrastructure improvements.
YMMV*
Here is my network configuration if it will help clarify my test descriptions:
Shown below is the image above as it was received as a fax that was sent over my Vonage voice line to my Vonage fax line. It has been resized and converted from a .tif to a .gif to fit the image display requirements of this forum. It was actually sent and received as a full 8.5 x 11 (US letter size) 2255 x 1728 px 200 dpi fax page:
For comparison, shown below is the image that was saved as a file on the transmitting fax server (a different PC than the receiving fax server, so these are not just duplicates of the same image) before it was actually sent as a fax. You will note that there is very little (if any) image degradation between the original image and the received fax image. It was consistent results of tests like this that convinced me that I could now rely on using Vonage for sending and receiving faxes and cancel my eFax account (which I setup several years ago after a failed attempt to get Vonage to reliably send/receive faxes).
I am not sure if Vonage handles Vonage to Vonage calls internally as IP to IP traffic or if it simply sends everything over the PSTN, but I did the same battery of tests in both directions between my Vonage line and my eFax service which would certainly have gone over the PSTN as analog faxes, and the results were the same. No dropped faxes and no degraded faxes in either direction. I have also not had any known problems with sent or received live faxes since I have started using my Vonage fax line.
Now that I have sung praises to the Vonage fax capabilities, allow me to once more post the obligatory warning:YMMV

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