ATT, T-mobile and VOIP

[nazzza]

Hi,
I have just bought a tilt unlocked to use with my T-mobile prepaid card.
I will use the device principally like GPS and VOIP calls, this is my first PDA and I bought the tilt for the triband 3g (for the voip calls) that I can use in europe too (I'm italian) and for the inbuilt gps.
I was stupid and just discovered that t-mobile has different bands for the 3g, and that my tilt will not work with it, more I can't do an ATT plan because I have no social security number. If I would have discovered this before I would have bought a cheaper edge symbian phone with inbuilt gps
Here the questions:
There is an other carrier for 3g that will work with my tilt?
I don't want to pay a data plans with T-mobile that I will not use: is acceptable the quality of voip calls with t-mobile or I need to switch to a different carrier?
Thank you for your help

[nazzza]

I've just fund here
att prepaid data (http://darlamack.blogs.com/darlamack/2008/05/att-prepaid-3g.html)
that att offers prepaid flat data, but I don't see nothing on their website

fund!
medianet (http://www.wireless.att.com/cell-phone-service/services/serviceDetails.jsp?LOSGId=&skuId=sku930040&catId=cat1360001)
I asked and replied by myself :)
only the last question: will be possible to use voip on medianet or ATT has some limit to voip?
is possible to set up media net and have "normal" access to all websites?