IMEI is the abbreviation for International Mobile Equipment Identity. It is a number unique to every GSM or UMTS mobile phone in the world. An IMEI number contains 15-17 digits. The IMEI number is used by the network to identify valid devices and therefore can be used to stop a stolen phone from accessing the network. The IMEI number is only used to identify the device, and has no permanent or semi-permanent relation to the subscriber.