Serial Number Mixmatch

Hi, I have been with you guys for years and my phone had gotten broken last year and I got a replacement sent to me. I tried to do any early upgrade and send the phone back in and you sent it back to me saying the serial numbers don't match when thats the phone you sent me. Now they are wanting me to pay the price of the phone, which I am not going to do. I want to send the phone in. I have put in multiple tickets and this has been going on for over a month with numerous trips to the xfinity store and as well as on the phone and online and it still hasn't been fixed. They are now threatening to turn my phone off if I dont pay it. I am not paying for something I am not responsible for. I am getting ready to take the situation to court if I need to for something to get done about it. I am a platinum member and I have had more problems then not out of this.