best solution is to get a tmobile blackberry or other UMA supported device.
i was in the same situation, and didn't want to use skype or a third party voip application, i wanted to have one phone number that i could be called or call on, 24/7.
tmobile allows UMA, which will use the wifi signal from your wireless router and use it as a cell tower.
i worked for apple and wouldnt use an iphone since at&t will not embrace this beautiful technology.