|This IMEI thingy?|
Every phone has a unique serial number. This number is called the IMEI number (International Mobile Equipment Identity).
You can get your IMEI by pressing "*#06#" (without the quotes) into your mobile. This will give you a (at minimum 15-digits) number. The first two refer to your country, the first six are known as the Type Approval Code. Then the seventh and eighth digits give who manufactured the phone (for example, 61 and 71 are Ericsson). The next six are your phone's serial number and the last digit is just an "additional number".
It is a good idea to record your IMEI number as most service providers (SP) keep a "ban" list and if the SP finds one of their subscribers using an IMEI in the ban list, the SP may decide to contact you to notify you that you may have purchased a stolen mobile or do not allow you to use the mobile with their network.