Typically when you purchase a cell phone, the company that sold you the phone (ex: AT&T) '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 if 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. Changing your SIM card to have your phone work on another network is useful especially if you want to travel to another country and want to buy a pre-paid SIM card so you can have cell phone access without roaming charges. There are services that offer to provide unlocking capabilities for a fee, but don't pay for them! You can do it yourself for free, which is the perfect price.