I was on Active Duty from June 10 2006 to June 15

I was on Active Duty from June 10 2006 to June 15 2010. During that time I was stationed overseas in South Korea. Before I left, I gave a copy of my orders to an official T-Mobile kiosk to have them terminate my account without incurring an early termination fee. About 4 months after leaving active duty, I started receiving phone calls from the Midland Credit Management company trying to collect a debt for T-Mobile. I then faxed them a copy of my orders showing that I was over seas and that under the SCRA I was covered for such a situation. They said that my orders aren't good enough and that they need a letter from T-Mobile stating that I have zero owed on the account. T-Mobile says that they can't do that because the debt has been sold to the collection agency. How do I go about solving the problem of getting this so called debt erased?