Iphone Cake

Most iPhones were and are still sold with a SIM lock, which restricts the use of the phone to one particular carrier, a common practice with subsidized GSM phones. Unlike most GSM phones however, the phone cannot be officially unlocked by entering a code. The locked/unlocked state is maintained on Apple's servers per IMEI and is set when the iPhone is activated.
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.