To check your mobile phone's serial number, key in the following digits on your phone *#06# (star,pound 06,pound) a 15 digit code will appear on the screen.  This number is unique to your handset.  Write it down and keep it safe.  If your phone is stolen, you can call your service provider and give them your code and they will be able to block your handset.  Even if the thief changes SIM cards or whatever, the phone will be totally useless to them.  So even if you don't get your phone back, you will be assured they can't use it either.