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The International Mobile Equipment Identity or IMEI is a number unique to every GSM and UMTS mobile phone.
The IMEI number is used by the GSM network to identify valid devices and therefore can be used to stop a stolen phone from accessing the network. For example, if a mobile phone is stolen, the owner can call network provider and instruct them to block the phone using its IMEI number. This renders the phone useless, regardless of whether the phone's SIM is changed.
Unlike the Electronic Serial Number or MEID of CDMA and other wireless networks, the IMEI is only used to identify the device, and has no permanent or semi-permanent relation to the subscriber. Instead, the subscriber is identified by transmission of an IMSI number, which is stored on a SIM card.
Example of the IMEI code 37-209900-176148-1
TAC: 372099 so it was issued by the BABT and has the allocation number 2099
FAC: 00 so it was numbered during the transition phase from the old format to the new format (described below)
SNR: 176148 - uniquely identifying a unit of this model
CD: 1 so it is a GSM Phase 2 or higher
SVN: 23 - The 'software version number' identifying the revision of the software installed on the phone. 99 is reserved
Final Assembly Code (FAC) For different vendor

01,02 AEG
07 , 40 Motorola
10, 20 Nokia
30 Ericsson
40, 41, 44 Siemens
47 Option International
50 Bosch
51 Sony
51 Siemens
51 Ericsson
60 Alcatel
70 Sagem
75 Dancall
80 Philips
85 Panasonic

Retrieving IMEI information from a GSM device
On many devices the IMEI number can be retrieved by entering *#06#. The IMEI number of a GSM device can be retrieved by sending the command AT+CGSN through data cable.
The IMEI information can be retrieved from most Nokia mobile phones by pressing *#92702689# (*#WAR0ANTY#), this opens the warranty menu in which the first item is the serial number (the IMEI). The warranty menu also shows other information such as the date the phone was made and the life timer of the phone.The IMEI can frequently be displayed through phone menus, under a section titled 'System Information', 'Device', 'Phone Info' or similar.
Computation of the Check Digit
The last number of the IMEI is called the Check Digit and is defined using a formula called "Luhn formula", thus sometimes the last digit is called the Luhn Check Digit and does not necessarily always set to 0.
According to the IMEI Allocation and Approval Guidelines,
The Check Digit is calculated according to Luhn formula (ISO/IEC 7812). The Check Digit shall not be transmitted to the network. 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 check digit shall always be transmitted to the network as "0".
This check digit is computed in three steps as shown in the following example.
Given 14 most significant digits of the IMEISV without the SVN which are the 6-digit TAC, 2-digit FAC, and 6-digit SNR and are labelled as follows:
TAC = D14 D13 ... D9 (with D9 the least significant digit of TAC);
FAC = D8 D7 (with D7 the least significant digit of FAC); and
SNR = D6 D5 ... D1 (with D1 the least significant digit of SNR).
Example IMEI: 49015420323751? (ignore the last digit)
TAC: 490154
FAC: 20
SNR: 323751
Computation of its check number / CHK for the IMEI proceeds as follows:
Step 1: Double the values of the odd labelled digits D1, D3, D5 ... D13 of the IMEI.

Step 2: Add together the individual digits of all the seven numbers obtained in Step 1,
1+8 + 2 + 8 + 0 + 4 + 1+4 + 2 = 30
and then add this sum to the sum of all the even labelled digits D2, D4, D6 ... D14 of the IMEI.
4 + 0 + 5 + 2 + 3 + 3 + 5 = 22
The result is:
30 + 22 = 52 or 4 + 1+8 + 0 + 2 + 5 + 8 + 2 + 0 + 3 + 4 + 3 + 1+4 + 5 + 2 = 52
Step 3: If the number obtained in Step 2 ends in 0, then set CHK to be 0. If the number obtained in Step 2 does not end in 0, then set CHK to be that number subtracted from the next higher number which does end in 0.
Since 52 ends in "2", CHK can't be set to "0". The next higher number that does end in "0" after 52 is 60, so we set: CHK = 60 - 52 = 8
So the IMEI number is: 490154203237518 and it ends in 8.