charged for the returned old phone, please help how i can get my money back?

i got a new phone and returned the defected old phone in a box with printed return-label by T mobile. They have still charged me for the returned phone almost 790$. I contacted t-mobile via chat, and complained, they issued me a complane reference ID. But never heard back from them, did not get my money back. I chatted several times, but every time the T-Mobile agent looks like never seen any complain from me. It is awful, it is a lot of money and I can't ignore. Please somebody share his/her experience and tell me how got the money back?. Thanks.