AT&T and Verizon Wireless are two of the largest cellular providers in the United States. One important distinction between the two carriers is that their devices use competing cellphone technologies – "GSM" and "CDMA." Consequently, it is not possible to convert or "flash" a Verizon Wireless cellphone for use on the AT&T network.
GSM Vs. CDMA
A number of technical differences exist between GSM and CDMA wireless technologies, but the most pertinent one to cellphone users is how the phones are linked to their networks. AT&T and T-Mobile provide customers with GSM phones that use a removable plastic "SIM" card to link the phone to its carrier. On the other hand, CDMA cellphones are tied to their carrier by digital programming sequences that are coded into the device.
Although you can, in theory, use a GSM phone on another network simply by removing its SIM card and inserting one from a different carrier, mobile service providers have made this process more difficult. Specifically, when you purchase a GSM cell phone, it comes "locked" to the carrier that sold you the phone. For example, if you purchase an iPhone from AT&T, it is programmed to work only on the AT&T network. To use a locked GSM cellphone with another carrier, you must "jailbreak" it, a procedure most carriers frown on and one that usually voids the cellphone's warranty.
CDMA phones don't use SIM cards or other removable media. To use a CDMA cellphone on another network, you need have it reprogrammed, or "flashed." One key difference between unlocking and flashing is that while you need only unlock a GSM phone once, and you can then change SIM cards as often as you'd like, you must re-flash a CDMA phone every time you want to use it with another carrier. You cannot flash a GSM phone to work on a CDMA network, and you can't unlock a CDMA phone and use it on a GSM network.
Verizon Phones on AT&T
Since AT&T's network accommodates only GSM devices, which require the use of SIM cards, unfortunately, it isn't possible to use a Verizon Wireless device on the AT&T network. If you currently use Verizon and want to switch to AT&T, your best bet is to shop for AT&T phones online and try to find one that is comparable to your Verizon cellphone. You can try to recoup the cost of your upgrade by selling your Verizon phone.