Iphone Cake

While the iPhone was initially sold in the US only on the AT&T network with a SIM lock in place, various hackers have found methods to "unlock" the phone from a specific network. Although AT&T and Verizon are the only authorized iPhone carriers in the United States, unlocked iPhones can be used with other carriers after unlocking. More than a quarter of the original iPhones sold in the United States were not registered with AT&T. Apple speculates that they were likely shipped overseas and unlocked, a lucrative market before the iPhone 3G's worldwide release. iPhones are unlocked by users who wish to use networks other than AT&T's.
AT&T has stated that the "iPhone cannot be unlocked, even if you are out of contract". On March 26, 2009, AT&T in the United States began selling the iPhone without a contract, though still SIM-locked to their network. The up-front purchase price of such iPhone units is often twice as expensive as those bundled with contracts. Outside of the United States, policies differ, especially in US territories and insular areas like Guam, where GTA TeleGuam is the exclusive carrier for the iPhone, since neither AT&T nor Verizon have a presence in the area.
• United Kingdom
Operators O2, Orange, 3, Vodafone, T-Mobile, as well as MVNO Tesco Mobile sell the device in the UK under subsidised contracts, or for use on pay as you go. They are locked to network initially, though are usually able to be unlocked either after a certain period of contract length has passed, or for a small fee. The iPhone 4 in the UK is also currently sold unlocked without a contract for full retail price for use on any network the user chooses, but only when bought directly from Apple (either online or in their physical shops).