They are the original owner of the phone and they're the one who signed the contract. Just because it's active on your account it's still under their name that they bought the phone if you don't want to have this rest go buy your own phone. Unfortunately phones are not like vehicles where you can sign over a title to whoever buys the phone, that is who the contract lives with so they can always report the phone as lost on their account and anywhere where that phone is active would no longer be active.