There are two major types of cellular phone technologies: GSM and CDMA. GSM stands for Global System for Mobiles. CDMA is the abbreviation for Code Division Multiple Access. U.S. network service providers, such as T-Mobile and AT&T, use GSM, while U.S. Celllular, Sprint and Verizon use CDMA.
A removable SIM card is used to store vital information for GSM carriers. Swapping an unlocked phone from one GSM network service provider to another is easy. For example, switching an unlocked phone from AT&T to T-Mobile is possible. CDMA carriers use network-dependent white lists to authenticate their subscribers, making prior permission from the CDMA network service provider is required for switching. Such permission is normally not given. For example, a Verizon phone without a SIM card cannot be switched to Sprint.Learn more about Mobile