How can I get negative balance back?

I have been a T-Mobile customer since 2014 until last November when I switched to another service provider. I really loved the service but I had no choice other than moving to another provider as there is no cell phone signals in my home. This is really a surprise that my phone is located in the same city where T-Mobile headquater is, Bellevue, Washington.

My monthly payment was $100 for two lines, old One Plan, with one free third line. I received a final bill in January that surprised me a lot because prorated charge was a way more than I thought. My charge was for less than a week but I got charged $16 for two lines and $36 for free line. How can this be $36 for the free line?

I talked to customer service a couple of times and my disaster began after an agent credited it back to my account. The agent took a shortcut instead of handling the issue properly to avoid hasstle. The agent told me to wait for check to be mailed as my account is closed. I trusted this and waited for a month but nothing happened. And realized after a couple of more calls with customer service that it is "credit" so it can't be refunded. How can I use this credit? The account is already closed!

I've talked to one of supervisors who treated me like a stupid baby, told me at the end of the call that he will handle this differently so it can be reflected in the next bill statement. Yes, he treated me as a stupid baby during the whole call and nothing happened. He told me that "OK, I'll handle this differently this time. You will receive a check via mail after the bill is processed. I'll follow this up and give you a phone call with an update once bill process is complete". I trusted this but realized after a month that this was a way to discontinue conversation with me.

If it can’t be refunded, can anyone let me know how to use this credit from closed account?