Typically when you purchase a cell phone, the company that sold you the phone (ex: Cingular) 'locked' the phone before you bought it, so you can only use it on their network. This means that you can't use your phone on any other network, even if you change the SIM (which is an abbreviation for Subscriber Identity Module) card. The SIM card is the little piece of plastic that identifies your phone to the cellular network.