Being charged for an old account after transferring to a new one?

A few years ago, I joined T-Mobile with my partner at a T-Mobile store. The representative gave me a 3rd line as part of a promotion for free. He said that my bill would be lower if I got this 3rd line, and I could decide to activate it for someone else in the future.

I never did.

Fast forward to last year, I transferred my line and my partner's line to a new account. A T-Mobile phone representative assured me that this 3rd line would be dropped, and I wouldn't be charged for it. 

I was.

For months, I was charged for this 3rd line on my old account. Every few weeks I reached out to T-Mobile, and they assured me that they fixed the account and refunded me the money charged.

They never did.

Month after month I was charged until finally, I received a letter from a Convergent Outsourcing collection agency. I was put on Collections. ¿Qué hago?