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 dialpad, or alongside other system information in the settings menu on smartphone operating systems. 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 blacklist 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). 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. When someone has their mobile equipment stolen or lost, they can ask their service provider to block the phone from their network, and the operator does so if required by law. If the local operator maintains an Equipment Identity Register (EIR), it adds the device IMEI to it. Optionally, it also adds the IMEI to shared registries, such as the Central Equipment Identity Register (CEIR), which blacklists the device with other operators that use the CEIR. This blacklisting makes the device unusable on any operator that uses the CEIR, which makes mobile equipment theft pointless, except for parts. To make CEIR blacklisting effective, the IMEI number is supposed to be difficult to change. However, a phone's IMEI may be easy to change with special tools. In addition, IMEI is an un-authenticated mobile identifier (as opposed to IMSI, which is routinely authenticated by home and serving mobile networks.) Spoofed IMEI can thwart all efforts to track handsets, or target handsets for lawful intercept. Australia was the first nation to implement IMEI blocking across all GSM networks, in 2003. In Australia the Electronic Information Exchange (EIE) Administration Node provide a blocked IMEI lookup service for Australian customers. In the UK, a voluntary charter operated by the mobile networks ensures that any operator's blacklisting of a handset is communicated to the CEIR and subsequently to all other networks. This ensures that the handset is quickly unusable for calls, at most within 48 hours. All UK Police forces, including the Metropolitan Police Service actively check IMEI numbers of phones found involved in crime, against the National Mobile Property Register (NMPR). The NMPR draws its information from many property databases. One of the databases consulted is Immobilise, which allows optional (and free) registration of devices by the public. Such registration ensures that a device coming into police possession may be easily reunited with its registered owner. In New Zealand the NZ Telecommunications Forum Inc provide a blocked IMEI lookup service for New Zealand consumers. The service allows up to three lookups per day and checks against a database that is updated daily by the three major mobile network operators. A blocked IMEI cannot be connected to any of these three operators. In Latvia the SIA "Datorikas institūts DIVI" provides a blocked IMEI lookup service for checks against a database that is updated by all major mobile network operators in Latvia. In some countries, such blacklisting is not customary. In 2012, major network companies in the United States, under government pressure, committed to introducing a blacklisting service, but it's not clear whether it will interoperate with the CEIR. GSM carriers AT&T and T-Mobile began blocking newly reported IMEIs in November 2012. Thefts reported prior to November 2012 were not added to the database. The CTIA refers users to websites at www.stolenphonechecker.org and the GSMA where consumers can check whether a smartphone has been reported as lost or stolen to its member carriers. The relationship between the former and any national or international IMEI blacklists is unclear. It is unclear whether local barring of IMEI has any positive effect, as it may result in international smuggling of stolen phones.

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