Most iPhones were and are still[citation needed] 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.[citation needed] The locked/unlocked state is maintained on Apple's servers[citation needed] per IMEI and is set when the iPhone is activated.[202][failed verification]