Simply write down on your mobile phone following code.
To check your Mobile Phone's Serial number, key in the following digits on your phone:All mobile phone users should keep a note of their handset's serial number (known as the 'IMEI'), as it's very handy if the phone is lost or stolen. The number can normally be found under the battery (looking something like 004400/01/123456/7).
You can also get this number from the phone software, by entering following usefull code

Simply write down on your mobile phone following code.

* # 0 6 #

A 15 digit code will appear on the screen after pressing send button, or in some phones it comes automatically just by writing above given code. . This Number is unique to your Handset. Write it down and keep it somewhere safe. Should your phone get stolen, you can phone your service provider and give them this code. They will then be able to block your handset so even if the thief changes the SIM card, your phone will be totally useless. You probably won't get your phone back, but at least you know that whoever stole it can't use/sell it either. If everybody does this, there would be no point in people stealing mobile phones