IMEI numbers

The IMEI comes out of the GSM standard, drafted by the Groupe Spécial Mobile inside CEPT from 1982 and transferred to ETSI, founded in 1988, as the specifications matured. Its designers made a decision that still shapes the format: the subscriber and the handset carry separate identities. The IMEI identifies only the equipment and has no particular relationship to the subscriber, who is identified instead by the IMSI stored on a SIM that can in principle be moved to any handset. The first GSM call was placed on Radiolinja's Finnish network on 1 July 1991, by the prime minister Harri Holkeri.

The structure has been rebuilt twice. Before 1 January 2003 an IMEI opened with a six-digit Type Approval Code, recording that a national body had approved the device, followed by a two-digit Final Assembly Code identifying the company that actually built it. The approval half of that arrangement had already been hollowed out in April 2000, when the European R&TTE Directive severed the link between regulatory approval and IMEI allocation — so for more than two decades a formally allocated range has not implied that a terminal is approved or compliant. From January 2003, as member nations moved towards manufacturer self-regulation, the Final Assembly Code was fixed at 00 on every phone; on 1 April 2004 it was abolished outright and the type code widened into the eight-digit Type Allocation Code that a manufacturer now requests directly. The often-repeated claim that the seventh and eighth digits of a modern IMEI name the assembly plant is therefore two revisions out of date.

The first two digits of that code are the Reporting Body Identifier, naming the GSMA-approved group that allocated it; BABT, the British Approvals Board for Telecommunications, holds 35. The authoritative register is the GSMA's Device Database, which is open to partners only — which is precisely why no public page, this one included, can promise that a generated code is unissued.

Key properties

• An IMEI is 15 decimal digits: an 8-digit Type Allocation Code, a 6-digit manufacturer-defined serial and a check digit. An IMEISV is 16 — the same 14 leading digits, then a two-digit Software Version Number in place of the check digit.
• The check digit is a function of the other 14 digits, computed with the Luhn formula of ISO/IEC 7812; the Software Version Number is excluded from that calculation, which is why an IMEISV does not satisfy a Luhn test.
• The Luhn formula catches every single-digit error and every transposition of two adjacent digits except 09 against 90.
• The first two digits of the Type Allocation Code are the Reporting Body Identifier, naming the GSMA-approved group that allocated the code; BABT in the United Kingdom holds 35.
• Before 1 January 2003 an IMEI began with a six-digit Type Approval Code and a two-digit Final Assembly Code; from then until 1 April 2004 the Final Assembly Code was 00 on every phone, after which it was abolished and the type code widened to eight digits.
• An allocated IMEI range has not implied regulatory approval since April 2000, when the European R&TTE Directive removed the link between the two.
• The IMEI identifies equipment rather than the subscriber, who is identified by the IMSI held on the SIM; a dual-SIM handset normally carries two IMEIs.
• A 15-digit IMEI sits below JavaScript’s exact-integer limit of 2^53−1 ≈ 9.007 × 10^15, but a 16-digit IMEISV does not, so this page handles both as digit strings.

Where they turn up

• Entering *#06# on the dialpad displays the IMEI on most handsets; it is also commonly printed inside the battery compartment and listed in the settings menu.
• An operator that keeps an Equipment Identity Register adds a reported-stolen handset’s IMEI to it, and may pass it to the shared Central Equipment Identity Register so that other operators block the device too. A voluntary charter in the United Kingdom aims to propagate a block within 48 hours; in the United States, AT&T and T-Mobile only began blocking newly reported IMEIs in November 2012.
• The claim that blocklisting makes handset theft pointless is weaker than it sounds. Unlike the IMSI, the IMEI is never authenticated by the network, and it can be rewritten with the right tools — which is why altering one is a criminal offence in a number of jurisdictions, the United Kingdom’s Mobile Telephones (Re-programming) Act 2002 being the standard example.
• New Zealand’s subsidised rural broadband initiative embeds a list of permitted Type Allocation Codes in the SIM card itself, so that the cheap data rate cannot simply be moved into an unapproved device.
• Mobile device management systems and asset inventories track company handsets by IMEI because, unlike a phone number or a SIM, it stays with the hardware.