IMEI number is structured by the BABT. It has 15 decimal digits. Actualy it has 14 digits plus a check digit. The structure of the IMEI is specified in 3GPP TS 23.003. First 8 digits of IMEI number are TAC (Type Allocation Code) which will give you the mobile phone brand and model. Other 7 digits are defined by manufacturer (6 are serial number and 1 is check digit).
If you would like to make sure that check digit is correct click here.
From 2004, the format of the IMEI is AA BBBBBB CCCCCC D.
In any of the above cases, the first two digits of the TAC are the Reporting Body Identifier, which identifies the GSMA-approved group that allocated the TAC. The RBI numbers are allocated by the Global Decimal Administrator. The first two digits are always decimal (i.e., less than 0xA0), allowing IMEI numbers to be distinguished from an MEID, which will always have 0xA0 or larger as its first two digits.
Example of IMEI number 35 780502 398494 2 structure:
35 is reporting body identifier.
780502 manufacturer and brand (phone type)
398494 serial number
2 check digit
See also check digit calculation.