I'm thinking about upgrade my parents' 6S with 7 plus from at&t ( their current carrier), and then wait 60 days to switch over to T-Mobile and they will pay off the remaining payments for them if I'm not mistaken or misunderstood. Question is at&t locks their iPhones, will only unlock if u pay off the phone, if that's the case then I can forget about my plan right? Or can T-Mobile unlock an at&t iPhone so it can use a T-Mobile SIM card?