My sister has an old iphone 6s laying around the house that has a cracked screen. She told me I can have the phone. What I want to know is can I take this phone which I believe was with Sprint(not sure if she paid off the contract) and take it and add it to my AT&T account for another line. I don't want to go and fix the screen and the phone and I can't use it with AT&T. I keep hearing that if the phone is locked by the old carrier it's no good.