I had (have) an old desktop that I was not using and decided to use it as a dedicated Magic Jack machine to route all of my calls through. I proceeded to put my old desktop on my existing network and access it via Remote Desktop.
One day, I decided I needed to access the old desktop Remotely while trying to setup my fax machine on the box as well. Well, low and behold, my Magic Jack was not working properly; meaning, I got a dial-tone but could not call out.
It took me a while to figure out the problem. Once I found out that in order for Magic Jack to continue working properly, I would not be able to access it Remotely while using the local resources on the box I was Remoting from. Once I changed the options to use it's own local resources, everything worked!
I'm guessing that the Magic Jack needs to use the local resources when Remoting in order to produce sound correctly.