The International Mobile Equipment Identity (IMEI) is a unique 15-digit serial number that identifies a mobile device on a mobile network. It is usually found printed inside the device, on the silver sticker on the back of the phone, under the battery pack, or on the box the phone came in. The IMEI number is different from a serial number and is used to identify and track devices for a number of purposes. The IMEI number identifies the device and its specifications, such as the model, manufacturer, and software version. It is used by carriers to identify a device on a mobile network and is often used by both carriers and law enforcement to block stolen or lost devices from accessing a network. The IMEI number is also used to maintain a blacklist of stolen mobile phones. If a phone is stolen, the owner can report the theft to their mobile operator, give them the IMEI, and they would then block the phone. This means that the criminal would not be able to use it, even if they put in a new SIM card, as the device itself gets blacklisted. The IMEI number is also used to check that every phone call or SMS sent is from a mobile phone authorized to use a mobile network.