International Mobile Equipment Identity

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.[10] 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.[11] In Australia the Electronic Information Exchange (EIE) Administration Node provides a blocked IMEI lookup service for Australian customers.[12]

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[13] provides a blocked IMEI lookup service for New Zealand consumers. The service allows up to three lookups per day[14] 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"[15] 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.[16][17] GSM carriers AT&T and T-Mobile began blocking newly reported IMEIs in November 2012.[18] Thefts reported prior to November 2012 were not added to the database. The CTIA refers users to websites at www.stolenphonechecker.org[19] and the GSMA[19] 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.[19]

It is unclear whether local barring of IMEI has any positive effect, as it may result in international smuggling of stolen phones.[20]

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 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.[21]

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 a grace period for registering.[22] These include Chile,[23] Turkey,[24] Azerbaijan,[25] Colombia,[26] and Nepal.[27] Other countries that have adopted some form of mandatory IMEI registration include India, Pakistan, Indonesia, Cambodia, Thailand, Iran, Nigeria, Ecuador, Ukraine and Lebanon.[28]

The last number of the IMEI is a check digit, calculated using the Luhn algorithm, as defined in the IMEI Allocation and Approval Guidelines:

The Check Digit shall be calculated according to Luhn formula (ISO/IEC 7812). (See GSM 02.16 / 3GPP 22.016). The Check Digit is a function of all other digits in the IMEI. The Software Version Number (SVN) of a mobile is not included in the calculation.

The purpose of the Check Digit is to help guard against the possibility of incorrect entries to the CEIR and EIR equipment.

The presentation of the Check Digit both electronically and in printed form on the label and packaging is very important. Logistics (using bar-code reader) and EIR/CEIR administration cannot use the Check Digit unless it is printed outside of the packaging, and on the ME IMEI/Type Accreditation label.

The check digit is not transmitted over the radio interface, nor is it stored in the EIR database at any point. Therefore, all references to the last three or six digits of an IMEI refer to the actual IMEI number, to which the check digit does not belong.

The check digit is validated in three steps:

1. Starting from the right, double every other digit (e.g., 7 → 14).
2. Sum the digits (e.g., 14 → 1 + 4).
3. Check if the sum is divisible by 10.

Conversely, one can calculate the IMEI by choosing the check digit that would give a sum divisible by 10. For the example IMEI 49015420323751?,

To make the sum divisible by 10, we set x = 8, so the complete IMEI becomes 490154203237518.