Every device with an integrated modem or phone has a distinct 17 or 15 digit number that uniquely identifies it. The International Mobile Equipment Identity, or IMEI, number can be used to check information about the device, such as the model and brand.
The quickest way to find the IMEI number of a mobile phone is by dialing *#06# on the keypad. The IMEI number of most smartphones is usually accessible via the settings menu, under "About" or "General." Most manufacturers also record the IMEI number below or under the battery, or on the bottom or back of the device.
GSM networks use the IMEI number to validate the identity of a device. If a device is stolen, the owner of the device can call the network provider and have the device blocked from accessing the network by adding the IMEI number to a "blacklist." If a device's IMEI number appears on the "blacklist," the device is rendered useless, whether or not the SIM card is changed.
An IMEI number has no permanent relation to the device's subscriber. However, device owners are advised to record the IMEI number in a secure location immediately after purchase as a safe guard in case the device is stolen.