I have an ATT IPhone. I want to flash it to another network, how do I do that?

The ATT bill was not paid, I would like to use the phone as a prepay

Sinc eyou broke contract and/or did not pay the bill, ATT will not unlock the phone… So… And since ATT is GSM, that means all CDMA carriers (verizon, sprint, altell, metro pcs, cricket, virgin mobile) are out of the question to0…

so…

your only option… Jailbreak it… Hack unlock it via cydia app… Get a tmobile prepaid sim OR straight talk prepaid sim… Enjoy… Have a beer… Party

Jailbreak it and it might work.
if you need a unlocked iphone, email me i have a few for sale or trade.