I did some research some months ago, and although the official MagicJack.com site doesn't tout support with things like home security systems or FAX machines, I have managed to get a FAX machine to send faxes around the US.
For the sake of mention:
You can have Magic Jack assign you a telephone number in a different state than that from which you reside, but I have experienced latency problems with the service - such can erode connection quality with tone based devices.
A wireless connection to the computer with your device can cause intermittent connectivity drops.
Other computers wired (or wireless) to your Router can use bandwidth, and cause choppy connections on your Magic Jack, thus problems with tone based devices.
On your FAX device, you will want to set the minimum connection speed (9600 baud is ideal), and turn off error correction.
I have successfully sent a 17 page FAX from Salt Lake City to New York City, but receiving is only a half a page at best per receive.
When you install your Magic Jack Device, disconnect any other USB devices (not keyboard or mouse) such as USB printers, cameras, etc, so that the Magic Jack device can configure the dialing device first, and then the secondary USB drive that has the Magic Jack software.
Your Internet Connection should be at least a 1.5 MegaBit connection - any slower and the service might not work right.
Your computer should be at least a Pentium IV 1.6 Ghz or higher, with 1 Gig of Ram.
You might think to bring up the Magic Jack Service and application in the Task Manager, do a right click on it, and make it a high-priority task/application.
Let me know if you have any success with receiving a full 3-page fax as well as sending.