CDMA vs. GSM Technology

• Theoretically, GSM cell phones hook up to their networks entirely on the basis of removable plastic Subscriber Identity Module, or "SIM" cards. In practice, however, the majority of GSM carriers in the United States "lock" their phones so that an extra layer of digital protection exists, one a cell phone repair specialist or skilled ordinary user must re-program the device to overcome. CDMA cell phones, on the other hand, work solely on the basis of this sort of internal programming.
  
  

SIM Cards

• SIM cards are worthless if you use a CDMA device -- CDMA phones are incapable of accommodating them. In terms of the iPhone, this means that if you want to use an iPhone you purchased from Verizon Wireless (the only U.S. carrier to sell the phone's CDMA variant as of April 2011) with a GSM carrier such as T-Mobile, it would be impossible. No method exists to convert GSM devices for use on CDMA networks.
  
  

Unlocking

• If you have a GSM iPhone you wish to use on another network, you must unlock or "jailbreak" the device, which erases the code sequence by which AT&T keeps the phone on its network. Doing so is an extremely simple procedure. A number of third-party iPhone blogs have created "jailbreaking kits" packaged into a downloadable software programs you transfer from your laptop or desktop to your iPhone. This procedure still doesn't allow you to use your iPhone on a CDMA network and voids your Apple warranty.
  
  

Flashing

• Flashing is a nickname given to the CDMA unlock procedure that involves resetting your phone's internal "firmware," a programming sequence which links it to your carrier. The primary difference between the flashing and jailbreaking is that while jailbreaking is a one-time procedure which permits you to use your GSM device with any and all SIM cards, you must re-flash your CDMA phone each time you wish to use it on another network -- firmware necessary to do so is unique to each CDMA network. No competing CDMA networking the United States accepts Verizon iPhones for flashing as of April 2011. In other words, if you use a Verizon iPhone, Verizon is the only network with which you can use the phone.