Recently AT&T has begun officially unlocking iPhones which are out of contract.
But I know this doesn't mean that all carriers are supported, because some carriers use different frequency bands and simply aren't compatible with iPhones. After this official unlocking process, which carriers can be used with which models of iPhone devices?
Best Answer
You will find an up to date list of carriers and frequencies on this list for the United States. For other countries see here.
You can use your iPhones as follows (source):
This means you can use your iPhones on any carrier on the 850, 900, 1800, 1900 MHz frequencies as a GSM phone, and access to data at the slow GPRS/EDGE speed.
For higher speed access you will have to look for carriers who provide the right technologies at the right frequencies.
This mean the iPhone will not work on networks operating in the 1700 MHz frequencies and it will not work on 1800MHz 3G (UMTS/HSDPA/HSUPA) networks as it only can use this band for GSM.