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iPhone GSM (any model) can work easily on AT&T's gophone services as well as T-Mobile's but with T-Mobile you will only be able to use edge. It will work for Boost mobie as well (which is prepaid GSM) and yes the iphone does work on that carrier. This requires an unlock BTW. As for the iPhone 4 CDMA you can have it on Cricket, MetroPieceofCrapService, and Sprint. This requires a flash unlock which is more complicated because you ahev to edit the phone's internal files and set the APN and other setting to that specific carrier. Keep in mind that CDMA version, if unlocked, will only allow you to use text, sms, and phone. The data will not as it is still being figured out by the Dev-team. Good luck :)