Tmobile is charging me for a phone i returned

I ordered a phone online and it never worked. We went to two different stores, they couldn't get it to work but would not take it as a return. Finally got return label online and sent it off. Long story short, even though I have proof the package was received they "can't find the phone" and so I am responsible for it. I am beyond livid. Have talked called customer service at least five times at this point, even received a credit one month for the payment they took and was assured that the phone was removed from my account. Got a call today telling me basically we can't find it in our warehouse so even though you did what we instructed you're on the hook for a phone that never even turned on and was returned as soon as possible. I see I am not the only one who has experienced this with T Mobile. They are basically stealing money from me.