US GSM carriers

In the About (iOS) or Status (Android) page, scroll down and check for a MEID, ESN, or IMEI number. If your phone has a MEID or ESN number, it uses CDMA, and if it has IMEI, it uses GSM . If you see both, that means your phone supports both GSM and CDMA networks.

All unlocked phones work on carriers that use GSM networks. Popular U.S. carriers that use GSM networks include AT&T, T-Mobile, Cricket, MetroPCS, Simple Mobile and Tracfone . Most foreign carriers also use GSM networks. You will need to use a SIM card from a GSM provider to get service for your unlocked phone.

Which Are GSM? In the US, Verizon, US Cellular, and the old Sprint network (now owned by T-Mobile) used CDMA. AT&T and T-Mobile used GSM. Most of the rest of the world used GSM.