Vox and the Supafone does work over 3G. Vox is a VoIP provider and 3G also uses Internet Protocol. I know of two people that have had success in doing so. However there is no 100% guarantee of call quality as 3G is not the same all over. Primarily the phone is designed for ADSL use and the Vox call center will not be able to assist in the set-up of the 3G thou, just ensure you have a good branded ADSL router with 3G built in as you need a RJ45 plug into the phone and a VERY good 3G signal. I am a dealer for Vox and will probably go that way soon as where I'm moving to might not have Telkom lines for ADSL yet and cannot be without my Vox number.
One-post wonders notwithstanding, a li'l while ago our adsl went titsup for over a week ...so we jacked the E220 into the MR3420 and basically got on with life. It worked for the most part; it wasn't perfect and sometimes we had to redo a call but hey, we were using an (unflashed for moar speed that I know of) E220 for our connection! So: take heed about the quality of your signal, you will very much want all you can get.