International Mobile Equipment Identity

GSM networks use the IMEI number to identify valid devices, and can stop a 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 blocklist the phone. This renders the phone useless on that network and sometimes other networks, even if the thief changes the phone's SIM card.

Devices without a SIM card slot or eSIM capability usually do not have an IMEI, except for certain early Sprint LTE devices such as the Samsung Galaxy Nexus and S III which emulated a SIM-free CDMA activation experience and lacked roaming capabilities in 3GPP-only countries. However, 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 is stored 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.

Dual SIM enabled phones will normally have two IMEI numbers, except for devices such as the Pixel 3 (which has an eSIM and one physical SIM) which only allow one SIM card to be active at once.

IMEI and the law

Many countries have acknowledged the use of the IMEI in reducing the effect of mobile phone thefts. For example, in the United Kingdom, under the Mobile Telephones (Re-programming) Act, changing the IMEI of a phone, or possessing equipment that can change it, is considered an offence under some circumstances. A bill was introduced in the United States by Senator Chuck Schumer in 2012 that would have made the changing of an IMEI unlawful, but the bill was not enacted.

IMEI blocking is not the only way to fight phone theft. Instead, mobile operators are encouraged to take measures such as immediate suspension of service and replacement of SIM cards in case of loss or theft.

The existence of a formally allocated IMEI number range for a GSM terminal does not mean that the terminal is approved or complies with regulatory requirements. The linkage between regulatory approval and IMEI allocation was removed in April 2000, with the introduction of the European R&TTE Directive. Since that date, IMEIs have been allocated by BABT (or one of several other regional administrators acting on behalf of the GSM Association) to legitimate GSM terminal manufacturers without the need to provide evidence of approval.

Blocklists of stolen devices

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 may do so, especially 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 blocklists the device with other operators that use the CEIR. This blocklisting makes the device unusable on any operator that uses the CEIR, which makes mobile equipment theft pointless, except for parts.

To make blocklisting 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.) Using a spoofed IMEI can thwart some 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 provides a blocked IMEI lookup service for Australian customers.

In the UK, a voluntary charter operated by the mobile networks ensures that any operator's blocklisting 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.

Some UK Police forces, including the Metropolitan Police Service, actively check IMEI numbers of phones found involved in crime.

In New Zealand, the NZ Telecommunications Forum Inc provides 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 blocklisting is not customary. In 2012, major network companies in the United States, under government pressure, committed to introducing a blocklisting 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 blocklists is unclear.

It is unclear whether local barring of IMEI has any positive effect, as it may result in international smuggling of stolen phones.

Blocklists of incompatible devices

As of the 3G cellular network shutdown in Australia, all 4G and 5G devices not approved for use in Australia by the carriers (Telstra, Optus, Vodafone) are IMEI and/or TAC blocked. Users can not request the carriers to remove their device from blocklist, even if they can prove that it is compatible.

Limitations

IMEIs can sometimes be removed from a blocklist, depending on local arrangements. This would typically include quoting a password chosen at the time of blocklisting.

Law enforcement and intelligence use

Law enforcement and intelligence services can use an IMEI number as input for tracking devices that are able to locate a cell phone with an accuracy of a few meters. Saudi Arabian government agencies have reportedly used IMEI numbers retrieved from cell phone packaging to locate and detain women who fled Saudi Arabia's patriarchal society in other countries.

An IMEI number retrieved from the remnants of a Nokia 5110 was used to trace and identify the perpetrators behind the 2002 Bali bombings.

Allowlists

Some countries use allowlists instead of blocklists for IMEI numbers, so that any mobile phone needs to be legally registered in the country in order to be able to access mobile networks of the country, with possible exceptions for international roaming and during the grace period for registering. These include Chile, Turkey, Tunisia, Azerbaijan, Colombia, Nepal and Egypt. Other countries that have adopted some form of mandatory IMEI registration include India, Pakistan, Indonesia, Cambodia, Thailand, Iran, Nigeria, Ecuador, Ukraine, Lebanon, and Kenya.

Prior to their merger with T-Mobile, Sprint in the United States used an allowlist of devices where a user had to register their IMEI and SIM card before a LTE-capable device could be used, despite no US law mandating it. If a user changed their device, they had to register their new IMEI and SIM card. This isn't the case with other CDMA carriers such as Verizon which only used allowlists for 3G (which was a requirement for CDMA). T-Mobile does not use an allowlist but instead a blocklist, including for former Sprint customers and select Sprint-sold devices compatible with the current T-Mobile network.

AT&T and Telus also use an allowlist for VoLTE access, but does not require IMEI registration by customers. Instead, phone manufacturers are required to register their devices into AT&T's or Telus' databases, and customers are able to freely swap SIM cards or eSIMs into any allowlisted device. This has the problem that imported phones and some non-imported phones such as older OnePlus models or select LTE devices with CDMA support (including models sold on Verizon or Sprint) will not work for voice calls even if they have the cellular bands for AT&T and Telus, and support VoLTE on other networks or via VoLTE roaming.

Public IMEI lookup services

Besides regulatory blocklists and allowlists maintained by network operators, consumers can verify the status of their own devices using publicly accessible IMEI lookup services. By entering the 15‑digit IMEI number (obtained via the *#06# code) into these tools, users can retrieve information about a device’s warranty period, original carrier, SIM‑lock status, and whether it has been reported lost or stolen. Many of these services decode the Type Allocation Code (the first eight digits of the IMEI) to identify the device model and provide details such as blacklisting due to theft or unpaid bills. These tools help consumers avoid purchasing blacklisted devices and confirm that a phone has not been reported lost or stolen before reselling it.