IMEI number is an unique number given to every GSM phone. It is different from phone number, which is based on the SIM card used. IMEI number of phone will be same even when using different SIM cards. IMEI number is a 15 digit number supposed to be unique to each phone.
When you purchase a new phone, it is recommended to write down your phone's IMEI number in a safe place. IMEI number is usually displayed when you type *#06# in phone dial pad. it is also displayed in phone battery chamber or in the box of phone.
How IMEI helps in tracking lost mobile phone ?
When a GSM phone is inserted with a SIM card and when it connects to mobile service provider, the IMEI number of the phone is transmitted to mobile service provider. So when your phone is lost or stolen and is inserted with a new SIM card, then IMEI is still transmitted to the service provider of new SIM card. So if there is a network of all service providers in a country offering to find where a phone with particular IMEI is used, then it can help to locate any stolen or lost mobile phones.
Even though, there are such network of mobile service provides in some countries, It is not available in countries like India where cases of lost or stolen mobiles are a lot. In UK for example, there is Central Equipment Identity Register (CEIR) which maintains a database of all stolen or lost phone IMEI and all the service providers in UK blocks access to phones which have IMEI in this database. So in effect this renders the phone useless. If similar database is set up with help of all service providers in all countries, it will help a lot in recovering stolen mobile phones.
What to do when mobile phone is lost or stolen ?
First thing to do on losing your mobile phone is to contact your mobile service provider and have the SIM card blocked. This is to avoid somebody else using your mobile number. Nowadays mobile is used as security check in different areas like banking, email etc. so if you don't block the SIM, the stolen person can impersonate you to get access to your email or online banking account. When blocking the SIM, you may be required to produce some proof of identity which you gave while taking the SIM card. After blocking the SIM, service provide will issue a new SIM card which can be used in different mobile. So you can retain the mobile number you had in your lost mobile.
Next you can file a FIR with police, they can help in recovering the mobile phone. If you live in a country having a database of stolen IMEI like the one mentioned above, it will easier to recover the mobile.