Difference between a iphone 3GS locked and unlocked. What is the difference? I am trying to sell my iPhone 3GS which is 5.1 or something.. I see people selling the iPhone 3GS unlocked and locked. What is the difference. I have changed phones since a year ago but am thinking of selling the iPhone 3GS since I do not use it anymore. there is a $100 difference between a lock and unlocked phone. Can I ask ATT to unlocked this phone then?