Ordinary cell phone service works so that the unique serial number in your phone (its ESN) is used by the cell phone service provider to identify who you are. The wireless service 'knows' that a particular phone's serial number is your phone and knows to send phone calls to that phone when someone dials your phone number. A GSM phone is not directly linked to you. Instead, it has a removable account card, called a SIM (subscriber information module) and this SIM has a unique serial number on it. See similar questions...

The SIM (Subscriber Identity Module) card (SIM Chip) is the brain of your handset. The SIM card is approximately the size of a credit card. The SIM chip is removed from the card holder and inserted into the back of the handset. It’s about the size of a postage stamp, with embedded circuitry on one side. When inserted into a GSM handset, the SIM when activated by Cable & Wireless facilitates mobile service on a particular GSM network. See similar questions...