IMEI is a unique 15-digit serial number that uniquely identifies a GSM or UMTS mobile phone. It consists of four parts and provides information, such as the manufacturer, to the mobile network. It is usually found printed inside the battery compartment of the phone, but can also be displayed on-screen on most phones by entering *#06# MMI Supplementary Service code on the dialpad, or alongside other system information in the settings menu on smartphone operating systems.
The IMEI is automatically transmitted by the phone when the network asks for it. A network operator might request the IMEI to determine if a device is in disrepair, stolen or to gather statistics on fraud or faults.
GSM networks use the IMEI number to identify valid devices, and can stop a stolen phone from accessing the network. For example, if a mobile phone is stolen, the owner can have their network provider use the IMEI number to blocklist the phone. This renders the phone useless on that network and sometimes other networks, even if the thief changes the phone's subscriber identity module (SIM).
IMEI is most commonly used in GSM and WCDMA (UMTS) phones. It is embedded in the phone, not the SIM card. CDMA phones use a similar type of number called an ESN.
Devices without a SIM card slot usually don't have the IMEI code. However, the IMEI only identifies the device and has no particular relationship to the subscriber. The phone identifies the subscriber by transmitting the International mobile subscriber identity (IMSI) number, which it stores on a SIM card that can, in theory, be transferred to any handset. However, the network's ability to know a subscriber's current, individual device enables many network and security features.