IMEI numbers of cellular phones connected to a GSM network are stored in a database (EIR - Equipment Identity Register) containing all valid mobile phone equipment.

When a phone is reported stolen or is not type approved, the number is marked invalid.

The number consists of four groups that looks this:

nnnnnn--nn-nnnnnn-n

The first set of numbers is the type approval code (TAC). The first two digits represent the country code. The rest make up the final assembly code. The second group of numbers identifies the manufacturer:

01 and 02 = AEG

07 and 40 = Motorola

10 and 20 = Nokia

41and 44 = Siemens

51= Sony, Siemens, Ericsson

The third set is the serial number and the last single digit is an additional number (usually 0).