billing dispute

I used the warranty on my phone to replace it. i have the new replacement and shipped the defective phone with the UPS label and box that Tmobile provided. I have UPS confirmation that the phone was delivered to Tmobile and now tmobile wants to charge me for the phone and says they never got it. They even told me to file a dispute with UPS. I can not file a dispute with UPS since i did not purchase the shipping and they have confirmation of delivery. Now tmobile wants to charge me $584. I have been dealing with 611 customer service since january. They cut my line off until i called again to tell them what happened and they are “investigating again. I need a better contact or a resolution to this on going issue. any suggestions on how to resolve or who to contact? the highest person i reached using 611 was senior account specialist Savannah and said there was no one else to speak besides them. I need help!