Sprint has a device that hooks to your wifi and spits out a cell signal. This could possobly he modded to work with st but I haven seen it happen
Another option is groove ip. A voip sevice like skype that dosent have video calling. What it dose have tho is the ability to call phones for free even landlines. The app cost money then sign up for google talk then in settings forward everything to google talk wnd use ur wifi at home. Cheeper than new phone.