T-mob does not seem to care that you use an iphone,,, I have a family plan with them and have four iphones on it. Their tech support even has iphone support. Slight drawback is that only Edge works on Tmo, their 3g freq is different than what the iphone can receive and visual voicemail usually does not work... but four iphones with unlimited messaging, data, and 3000 minutes talk shared is ~$200/month.