Contract ****s

I have contract with MTN, last month I was later with my debit order but made arrangements to pay it off , which I did but month end I was debit again and that debit didn't go through and now I am being made to pay for they failed debit order that was not my contract and everytime when I call to acquire about it, I keep getting different story and no straight answers as to why that debit was placed under my name but they want the money for the fail order. I have sent emails with all the proof that it was they fault.