CDMA vs GSM

The two leading digital mobile phone technologies in the United States, and some parts of the world, are CDMA and GSM. These are competing technologies, "CDMA vs GSM," that are not compatible nor interchangeable. They're the reason you can't use AT&T phones on Sprints' network or Verizon’s and vice versa.

CDMA (Code Division Multiple Access) and GSM (Global System for Mobiles) are shorthand for the two major radio systems used in cell phones. Both acronyms include the basis for a set of expanding technologies and standards.

As of this writing the CDMA standard is the most expansive in the United States, with the most towards and cells. However, in Europe and most of the rest of the digital world the GSM standard is dominant. Following that are less developed countries still using the aging AMPS analog standard.

Verizon's phones and network are CDMA, which doesn't use sim cards. The new G4 phones do have a SIM card tied to the owner.

G3 and earlier CDMA phones do not use sim cards, while GSM phones all do.

CDMA and GSM are different technologies that are incompatible with each other.

The term "unlocking" referring to an unlocked phone is used when discussing GSM and iDEN phones.

There are other CDMA providers besides Verizon, which include Sprint and US Cellular.

Verizon will only activate another Verizon phone. A non-Verizon CDMA phone's ESN number will not be in Verizon's database of phones, and therefore, Verizon will not activate it. Verizon will also blacklist a Bad Verizon ESN Number on a formerly valid Verizon phone for certain reasons, which brings attention to one important caveat when buying a used Verizon phone.