All about IMEI

International Mobile Station Equipment Identity

(Here Mobile Station means the handset + SIM) 

• It is a unique number to identify mobile phones.
• mobile phones  i.e.,  3GPP (i.e., GSM, UMTS and LTE) and iDEN mobile phones, as well as some satellite phones.
• One can find the IMEI number in different ways, this link here provides you with the ways to find the IMEI number for different handsets. (I dialed *#06# and checked) 
• It is related to the device only, not to the subscriber who is using it. 
• These IMEIs are allocated by BABT
• It includes information on the origin, model and serial number of the device (note - this serial no. is different to the one which we get while we buy a phone)

Structure of IMEI Number

AA	-	BB	BB	BB	-	CC	CC	CC	D or EE

AA - BB BB BB : TAC (Type Allocation Code) 

• These represent the origin and the particular model of the device
• The first two digits are the RBI and the rest makes up the allocation number
• Reporting Body Identifier - indicates the GSMA-approved organization that allocated TAC 
• find the list of RBIs in the Annex A (page 19) of the doc stated at the end
• Earlier TAC used to be only six digits followed by FAC
• Final Assembly Code 
• was a manufacturer-specific code indicating the location of the device's construction (prior 2002) 
• was 00 (January 1, 2003 until that April 1, 2004 = transition period)
• are terminated to make TAC a 8 digit code (after April 1, 2004)
• Go through few TAC examples given here

CC CC CC : SNR

• Serial Number that distinctively identifies the unit of a model 

D or EE : Luhn check digit / checksum

• For the meaning of checksum, follow this link
• used to validate the IMEI number. 

Luhn Algorithm:

• It is used to validate a variety of identification numbers, such as bank account numbers, credit and debit card numbers, IMEI numbers etc. 
• Steps to calculate the number
• Moving from rightmost digit to leftmost, double the value of every second digit; if the product of this doubling operation is greater than 9 (e.g., 6 × 3 = 18), then sum the digits of the products (i.e., 1 + 8 = 9)
• Compute the sum of the digits
• Take the units digit
• Subtracting it from 10 results in the check digit.
• The above mentioned method is the easiest way, one can also verify the checksum by checking whether the total modulo 10 of the sum of the digits is equal to 0 or not. 

EE represents the Software Version Number identifying the revision of the software installed on the phone. The number sounds IMEISV in this case.

Theory done !! Practical part now !! 

I dialed *#06# on my phone

I will take the first one

352251065012588
TAC	35 - 22 51 06
RBI	35
SNR	50 12 58
Checksum	8

  

Now the verification of the checksum (Luhn algorithm)

Taking the first 14 digits = 3522510651258

3	5	2	2	5	1	0	6	5	0	1	2	5	8
3	10	2	4	5	2	0	12	5	0	1	4	5	16
3	1	2	4	5	2	0	3	5	0	1	4	5	7

The sum of digits equals 3 + 1 + 2 + 4 + 5 + 2 + 0 + 3 + 5 + 0 + 1 + 4 + 5 + 7 = 42

Unit digit number - 2 

So, the check digit is 10 - 2 = 8 (here it is) 

*** Interesting thing ***

Know the quality of your phone using the IMEI number

• Look at 7th and 8th digit, these will tell your handset quality
• Check this table

7th & 8th Digit	Manufactured at	Quality
00	Original Factory	Best
01/10	Finland	High
13	Azerbaijan	Too bad
02/20	UAE	Very Poor
03/30 & 04/40	China	Better
05/50	Brazil/Finland/USA	Good
06/60	China/Hong Kong/Mexico	OK
08/80	Germany	Fair

I am not sure about how much true it is but found this information in many websites so i included it.

Blocking IMEI Number

People ask the operator to block the SIM card once their mobile is stolen. What do they do exactly is blacklist the IMEI number. Before i write further i wish to say one thing i.e., IMEI is handset specific it becomes active once SIM is inserted.

Blocking of IMEI number makes the handset unfit to use particular operator's SIM which means one cannot put the same operator's SIM, they can try using other operator's SIM card in same state or other state also.

So when a person lodges a complaint that his mobile is stolen, what police does is that he provides the IMEI number to all the service providers throughout the nation and tell to check to which VLR is it latched to, from this he can identify under which BTS he is and go and catch him. It is also used to trace a particular MS.

NOTE: For CDMA handsets the counterpart is MEID

That is it, it sums up all the info i gathered about IMEI. Please refer this doc to read more

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Satya Sravan