IMEI (International Mobile Equipment Identity) is unique no. for every mobile inclusing GMS, CDMA, Satellite phones etc. It is printed on hardware of mobile, normally can be found near at battery box and can be checked by dialing *#06# from any phone.
As per govt. rules, every mobile requires unique IMEI no. to run and by govt order all mobile network blocked mobiles that do not have this code by 30 November, 2009.
There are some IMEI issues with cheap Chinese phones that comes without genuine IMEI no so govt ordered to block them. DoT issued a notice to “all customers to get themselves a genuine IMEI number from DoT approved service centers.” With help of IMEI no. we can track lost mobile phones and get their location and other information with help of network providers.