What is IMEI or Phone Identity Number, The International Mobile Equipment Identity or IMEI is a number, usually unique, to identify 3GPP and iDEN mobile phones, as well as some satellite phones. 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# on the dial pad, or alongside other system information in the settings menu on smartphone operating systems.
The IMEI number is used by a GSM network to identify valid devices and therefore can be used for stopping a stolen phone from accessing that network. For example, if a mobile phone is stolen, the owner can call their network provider and instruct them to blacklist the phone using its IMEI number. This renders the phone useless on that network and sometimes other networks too, whether or not the phone’s subscriber identity module (SIM) is changed.
The IMEI is only used for identifying the device and has no permanent or semi-permanent relation to the subscriber. Instead, the subscriber is identified by transmission of an International mobile subscriber identity (IMSI) number, which is stored on a SIM card that can, in theory, be transferred to any handset. However, many network and security features are enabled by knowing the current device being used by a subscriber.