Harassing me for their mistake

The app showed I had no data left, I bought airtime to buy more data. When I went into the app again to buy data, it showed that I still have data left, so I did not use the airtime to buy more data. When I looked again, all my airtime disappeared, because I did not have any data left.<br> <br> They did not debit my account on time. I left enough money in my account up until the 3rd of January, but still they haven't debited my account. Now they harass me every day, because I did not pay. They say there was a bank error. They waited till I don't have any money left to call me and sort it out. Now I have to pay a penalty of R100, because they waited too long. I have many debit orders on my account and can't keep track of all of them. Now they have also ruined my perfect credit record. I have told them they can debit the outstanding amount at the end of the month, but they still harass me every day.