unhappy with TELKOM

I have an account at Telkom and they usually debit my account every month end, but since month end September they have stopped debiting my account and I was also unware. I always make sure that I have funds prior telkom debit order that is why its so annoying to receive a call from them and tell me that I owe them, it is not my fault that they did not debit my account. I would appreciated if they can debit the outstanding amount this coming month end and stop calling me