The majority of iPhones are sold with a SIM lock, which restricts the use of the phone with one particular carrier, a common practice with subsidized GSM phones. Unlike most GSM phones however, the phone cannot be officially unlocked via inputting 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 on the AT&T and O2 network only with a SIM lock in place, various hackers have found methods to "unlock" the phone from a specific network. Although AT&T is the only authorized iPhone carrier in the United States and O2 is the only authorized iPhone carrier in the United Kingdom, unlocked iPhones can be used with an unauthorized carrier after unlocking. More than a quarter of the original iPhones sold in the United States and United Kingdom were not registered with AT&T or O2. Unlocking the iPhone is done in the U.S. and U.K. because many would-be users dislike switching carriers or consider AT&T's or O2's monthly fees too expensive.