In my case: 1) First I had a rough time cancelling my account _in person_ at a T-mobile store. The remote T-mobile employees in charge of cancellation kept hanging up on the T-mobile employees calling them from the store because cancellations are bad. This was so normal to the local T-mobile employees it was laughable to them. 2) T-mobile never cancelled my account - they suspended it...and didn't tell me! They reopened it 6 months later, charged me for a few months w/o notifying me, then sent the unpaid dues to a local debt collector. I only found out after being contacted by a debt collection agency.
I was able to get both T-mobile and the collection agency they work with to "look into it", but I could not get a direct answer from T-mobile about how to fix the situation after multiple calls to them, and the collection agency relied on T-mobile to strike the debt clean.
Bob Ferguson is the attorney general for Washington where T-mobile's HQ is. Bob is THE man in case you were wondering. After filing a complaint to his office, which was then forwarded to T-mobile, I heard back from someone at T-mobile specializing in these situations in a week's time and was informed the situation was fixed and the debt was removed from my credit report.