Is it possible to make WLAN enabled mobile use SIP through mobile broadband dongle?

Hi,

I recently moved to a new flat and decided to get Mobile Broadband (Orange) rather then a wired one. It works fine, and the dongle is obviously wi-fi enabled since it is able to connect through hot-spots where available.
Also, Orange are enlightened enough to not block VOIP. So, I am wondering, would it be possible to make a WLN mobile (e.g. Nokia E51) work with this set up? There is no router, but if the phone and the dongle can hook up in ad-hoc mode, then I get the internet through the dongle and use the WLAN on the mobile to make international calls through VOIP.
Wondering if anyone has got a similar setup to work, or knows the reason that it would not be possible? Thanks a lot for any advice. It would really save me a bundle, since I spend a lot of money on international calls every month...

If not, I guess I could always swap SIMs between the dongle and the HSDPA phone when I want to call on VOPI

AJ