Mtn trying to **** me

I have a contract with Mtn. Since end of September they keep trying to bill me for something I don't know, they are charging me an extra 1000 they can't even explain mainly because they can't see an invoice for August. How is that my problem when I make sure money gets debited each month end. Everytime I call them they say they logged the complaint, logging a complaint for months with no solution. Now I can't even cancel their contract that's ending this week because they refuse to solve their mess