The IMEI (International Mobile Equipment Identity) number is a 15 digit serial number unique to each and every GSM phone. A GSM phone has a SIM Card and is usually AT&T or T-Mobile.
You will usually find it printed on a sticker on the back of your cell phone or on the SIM tray of most Apple iPhones.
Alternatively you can have your mobile display its IMEI number by typing the following on the keypad: *#06#
If your handset displays more than 15 digits or is separated by symbols, please enter the first 15 digits only.