Vendors lock down devices before shipment to maximize their profit by making it difficult for users to switch carriers. For example, AT&T might lock a smartphone so that a user cannot easily switch to Verizon (or vice-versa). When corporations issue mobile devices to their employees in the corporate owned personally enabled (COPE) model, locking down each device can help to control support and management costs by discouraging unauthorized device exchanges among employees.
Some people try to unlock their smartphones so that they can accept SIM cards other than the original. Proponents of the practice argue that consumers should be free to change carriers when a subscription contract expires without having to discard hardware that they purchased outright. Nevertheless, smartphone unlocking was declared illegal in the United States in January 2013, overturning an earlier decision by the Copyright Office. The law applies to all devices purchased on or after October 28, 2012.
Device unlocking differs from jailbreaking, which involves removing certain manufacturer or carrier restrictions from a device, such as running a privilege escalation attack to alter the manufacturer's OS (operating system) by installing a custom kernel.