All US Carriers at least allow unlocking as long as you own the phone. Emphasis on own (paid off your contract so you are no longer "renting" it or you paid for

All US Carriers at least allow unlocking as long as you own the phone. Emphasis on own (paid off your contract so you are no longer "renting" it or you paid for it outright). That would include all GSM phones and all Global phones on CDMA based carriers (such as the Galaxy S3 or iPhone 5).