account I paid through them

I received a phone call from RPM at the end of 2010 about an old T-Mobile account I had. They gave me all the correct information about the account so I paid them over $2800.00 to pay off my bill. I am now receiving letters from another company through T-Mobile saying my bill was not paid. Now when I try reaching RPM about my bill and trying to get any kind of letter stating that it was paid in full I can not reach them in any way. All I get is a busy signal or a full voice mail. Fortunately I have all of my bank statements showing I paid them as well as all of their phone numbers to reach them at. However, I still have never received a confirmation letter or e-mail. I am not quite sure what to do about this as I have never been put in such a horrible situation before. I did however have to close my bank account to prevent them from taking any further funds out.