Help Settling A Collections Debt!

I've also been turned over to a collection agency, however I don't owe T-Mobile any money. I've spoken to customer service for four months, one to two times a month. Each time they admit the mistake is on their part. Each time the promise me that they will fix the bill.

I last spoke to them 6 days ago and they promised it would be fixed in 10 days. 

Today I got a call from a collections agency. I called T-Mobile and they are telling me "so sorry, you've been written off, we can't do anything." 

I talked with Joseph on the 30th of November

I talked to Mare on the 19th of December

I talked with Nicole, a supervisor on the 16th of January

I talked with Charles on the 16th of February at 1 PM and he promised a level 2 escalation was open and this would be fixed.

Today, I received a call from the collections agency. I immediately called T-Mobile and have been told now that my account is written off, they can't do anything.

I feel like they just stalled until they could write off my account. Then never really planned on helping me at all.