Your mobile IMEI can be found by pressing *#06#, always keep a note of your mobile’s IMEI number in a notebook or at some safe place, so that you can use it in case the mobile is lost.
When ever a mobile phone is switched off and on, the SIM card sends some internal information to the mobile network operator such as mobile IMEI number, mobile model number, mobile version and other technical details.
So if you have lost yourmobile phone you can contact your network provider to block your IMEI number.
First go to Police and get a FIR copy and then call customer service and tell to block your IMEI number.
All the network providers in India use a shared hosting service for blocking IMEI numbers, So if you tell to one network, all other networks will block that IMEI number automatically.