T-Mobile refuses to credit my account for a device I returned & is now threating collections

This is going to be a but long but I need help

I have 5 lines with T-Mobile for my business account. Last August I had some issues with one of my Samsung s24 and went to the the store for help. It was not charging as fast as it should so I was concerned. They requested a replacement device for me and it was shipped out. It arrived September 20th but was sitting in the mailroom for a while so I didn't know it was there so I went on vacation. I came back from vacation in late October and got the phone. By this time my current 24 phone was charging at normal speed so I decided not the take the replacement device. 

I sent the device back in November (full transparency, I kept forgetting to take it back to the store) so It was done mid November. I called customer service and checked and they told me to use the return slip that came with the package. I didn't open the package; it was sent back the same way it was sent to me. I was told I would get a confirmation email once they received it so I thought that was that. 

At the end of November, I got my bill and I saw a 1300 no-return charge for the device. I called customer service and I had to explain to them that the device I sent back was the one they sent to me, not the one I had issues with. I was told that once the phone was registered by the team they would credit the account. Another 2 weeks passed and nothing so I called them back. This time I was told that I had to wait until the warehouse finished sorting as they hadn't gotten confirmation that the device was received.  In Mid- December I called again and this time I was told that I would not be getting my credit because I didn't send the phone back. I had to get the tracking number from UPS and give them my IMEI number so they could verify that I still had my original device. I was told a ticket would be raised and that they would get back to me in 3 days. No one did so 5 days later I called again and this time I was told by a different agent that since I sent the device back after the grace period I would not get my credit (I thought it was 60 days and it was 45 days). I asked at this time if they would send the phone back to me since they are charging me the full cost of the phone. Again I had to go through the verification with my IMEI and only at this time did the rep confirm that they still have not received confirmation from the warehouse.  I had to ask for a supervisor and I was told that this is a unique case since I sent back the working device and not the one I wanted to replace. Again I was told another ticket would be made but this time I was asked to pay the 1300 charge and that it would be credited to future bills. Since I was told different things at different times I refused to pay it. 

This was the end of December and since then I have called at least twice a week for the last two & half months. I have been told the same thing and just sent to multiple people without any resolution. When I ask about the ticket I am told they will check but refuse to give me a reference number or tell me it's the internal ticketing system so they can't share the ticket details. At times I am on the phone with the rep for up to 2 hours just being sent around or waiting

 Note that my account was suspended in January even though I was told they were working internally to resolve this and I was calling them multiple times. I was then sent a notice that I have to pay for all the devices I had on my plan (so all 5 lines) I don't have an issue with this. My issue is that they are still asking me to pay the full cost for the returned phone but will not confirm if I will get it sent back or if I can go to a store for another. I have asked what will be done to the phone since I am essentially paying for a brand-new phone that is in T-Mobile's possession and I have not given a clear answer. 

Now they have threatened collection because I refuse to pay for the phone I returned in November. Yet I cant get an answer from customer service on the several tickets they claimed to have raised or the internal escalations they claimed they did to get my account credited. 

What do I do at this point?