IMEI (International Mobile Equipment Identity) is 15 digit or 17 digit (IMEISV) unique number for GSM and WCDMA mobile phones, used to identify the device. Every GSM device such mobile phone, GSM modem or device with a built-in phone / modem has this unique 15 digit IMEI number. The IMEI is only used for identifying 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 that can (in theory) be transferred to any handset. However, many network and security features are enabled by knowing the current device being used by a subscriber.
For example, the old style IMEI code 35-209900-176148-1 or IMEISV code 35-209900-176148-23 tells us the following:
- TAC: 35-2099 – issued by the BABT (code 35) with the allocation number 2099
- FAC: 00 – indicating the phone was made during the transition period when FACs were being removed.
- 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.
- Usually IMEI is printed in a label under the battery compartment with many other technical details, and this is hard-printed IMEI.
- To read IMEI on screen type *#06#. Onscreen keypad phones first bring the keypad onscreen and type code *#06# phone will display IMEI on screen, this is soft-coded IMEI
The second method is preferred to get IMEI for code calculation, because usually phone uses programed IMEI to check the entered unlock code.
Read more information http://en.wikipedia.org/wiki/International_Mobile_Equipment_Identity