Paid up letter

I had an account with mtn in which I lost my job, I couldn't pay my account. Mtn handed me over to Shapiro, we made an agreement that I will my monthly installment with until I pay up my account in which I did. I payed up my account I think in July or August. When I call them for my paid up letter I was told to wait 40 days and that was in September till today I haven't received my letter and mtn still says I owe them money and that they want proof that I paid up my account. Mtn has handed over my account to other 2 more debt collectors. It's a nightmare because I am getting calls each and every day and when I tell them that my account is paid up they want proof that it is paid up and I can't even provide it. I call Shapiro and their agents are very rude as if they are doing me a favor. I am super irate.