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If you have a phone that is capible of both cdma and gsm you can unlock your phone for att.
First, if you ask for an international unlock they will only unlock that portion of the gsm slot. They have fixed that little "error" some time ago.
Second, There was a recent law passed (jan of 2013 i think) that the carrier has to check to see if the device is active with another carrier. This i believe is to keep someone from stealing a phone and switching it for thier own use. This has hampered unlocking. Mostly Hampered :)
Lastly, if they refuse to unlock your phone, go get a gevey chip. It sits on the bottom of the sim and is a "plug and play" device. If you dont want it anymore you simply remove it and reset phone and no one is the wiser :)
First off, if it is a Sprint phone it is CDMA, so it is not compatible with AT&T or T-Mobile, which have GSM networks, no matter what you do. There are a few exceptions - a few phones are both CDMA and GSM compatible, but the only ones I can think of off the top of my head are a few BlackBerry and earlier iPhones models. Maybe it was one of those in the thread Christopher was reading, but if not, I call shenanigans on whoever was the original poster.
Second, you would need to get Sprint to unlock the phone, which they may or may not agree to do. But maybe you will get lucky with this.
Third, even if you get it unlocked, other carriers may refuse to activate it anyway. You might want to call the carrier you are considering and ask them if they will activate an unlocked device before even worrying about it.
Sprint will unlock your phone if you ask it to, but the company says it then can only be used internationally. But here's a comment from someone in a Sprint community thread:
"I will pass on a bit of information the local Sprint Store Manager told me. Even though Sprint tells you it is unlocked for international use only - it will indeed work domestically with other carriers. He said they are told to pass that on and many who pass it on believe it but he said he tried an AT&T Sim in one of the Sprint unlocked phones as an experiment and it logged into the AT&T network just fine and he even made a test call to see if it would connect."
Obviously I can't guarantee this will work, but that's what this commenter reports.