There are two main types of cell phone technology as of November 2010: CDMA and GSM. If you use a GSM device, you can change carriers by simply inserting a new, plastic SIM card into your phone. CDMA phones, on the other hand, require a "flash"--in other words, a reconfiguration of their internal programming--to be used on multiple wireless networks. Keeping this in mind, the process of "unflashing" a CDMA phone is more straightforward than you might think.
Discontinue any service you've established with the carrier from which you wish to unflash. While you will not be able to use a phone with, say, Verizon if it's not flashed to that carrier, Verizon (or whichever carrier you use) can still bill you for service that is connected.
Flash your phone to the new carrier of your choice. As CDMA phones must be programmed specifically to work on any network, the only means of unflashing a CDMA phone from a given network is to flash it to another. If you want to use carrier MetroPCS or Cricket, you may take them to a respective retail store in order to flash them. Otherwise, contact a cell phone or computer repair specialist in your area to complete the process.
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Establish a new line of service with your desire carrier. You won't be able to use your "reflashed" phone with your new carrier until you have established an account with them.