Competing Technologies

• Today, two main cell phone technologies compete for market share around the world: Global System for Mobile Communications or "GSM" and Code Division Multiple Access, better known as "CDMA." Both technologies are represented in the American mobile market. Carriers likes AT&T and T-Mobile use GSM technology, which functions on the basis of insertable SIM cards, on their networks. CDMA, which, uses no SIM card and is entirely based on internal programming, is the choice of not only Verizon, but also carriers like Sprint, MetroPCS and Cricket.
  
  

iPhone Carriers and Jailbreaking

• Prior to February, 2011, AT&T was the only carrier in the U.S. to have sold the iPhone. This didn't mean you couldn't use the device on another network. In order to use your AT&T iPhone -- or, indeed, any GSM device -- on another GSM carrier, you need only unlock or "jailbreak" it, a procedure which removes the "lock" the original carrier placed on the device. Although a relatively simple procedure, jailbreaking isn't endorsed by any telecommunications company and actually voids your iPhone's warranty.
  
  

Jailbroken Phones and CDMA Networks

• Once you jailbreak your AT&T iPhone, you may use it on any GSM network -- domestically or internationally -- simply by inserting a different carrier's SIM card into it. This being said, there currently exists no means of re-programming a GSM devices, unlocked or otherwise, to use it on a CDMA network. As a result, it isn't possible to use your unlocked AT&T iPhone with Verizon Wireless service, regardless of whether or not you've jailbroken the device.
  
  

Verizon iPhones

• As of May, 2011, Verizon is the only CDMA carrier in the United States to sell or support the iPhone. As a result, the only way to use an iPhone on the Verizon network is to purchase the device through Verizon or Apple alongside a Verizon iPhone service plan. Although a procedure approximately analogous to jailbreaking -- known as "flashing" -- does exist for CDMA devices, its usefulness in converting non-Verizon iPhones for use on the carrier's network is null until Sprint, MetroPCS or another domestic CDMA carrier begins selling their own iPhone variant, an event that may or may not ever occur.