What's the difference? GSM phones use subscriber identity module (SIM) cards. A SIM card is a small card that's inserted into the phone. It contains all your contacts and settings, and it's linked to your account. You can take the SIM card out, put it into another phone, and if someone calls your number, the new phone will ring. You can also put a different SIM card in your unlocked phone, and your phone will then work with whatever phone number and account is linked to that card. CDMA phones have no SIM cards and must be authenticated by the service provider, which makes unlocking a phone impossible [source: Segan ].