T-Mobile says no phone received in warehouse

Last July, I had a phone that wasn't charging, I was sent a replacement and was told a charger would come with it, there was no charger so I went to the Tmobile store and bought one just to find out that the original phone worked with that charger so I sent the replacement phone back. When my October bill came, there was a charge for the full amount of the phone on there. I called and was told it would have to be paid because the phone wasn't returned. I paid it at the time so my service wouldn't get shut off Now, after 4 months of fighting with Tmobile and being promised a refund(they finally found the phone I returned), I am being told that it is a valid charge because I didn't return the original phone, so bottom line is I have a monthly payment for the phone plus I already paid the full amount, and for my 4 months of being lied to and countless hours on the phone, I can have $100 credit on my bill. I guess they would rather lose money over the long run when I switch carriers instead of fixing this because I made the huge mistake of sending them back a brand new phone that they can resell. And on top of that, it took them until today to tell me that was the reason they wouldn't refund it.