Terriibly disappointed by beach of contractual agreement by Telkom

I have a fibre contract with Telkom. In the contract I indicated that the debit order should go off on the LAST DAY OF THE MONTH. However, this month Telkom decided that they will debit before 25 Dec. Unfortunately for them there were not enough funds available because I will only get my salary on 31 Dec. I find it annoying that the company, without any valid reason and without consulting me changes our contractual agreement which will result in me being financially penalised for Debit Order reversal as well as dent my credit ratings. These things have negative and far reaching implications for me and my family. Worst of all they complain that the debit order was not successful and I need to deposit urgently to avoid service disruptions. What they forget is that I indicated when they should debit my account and when they deviate from our contractual agreement they threaten service disruption. I am not fazed by that threat as I am currently looking for a service provider who will not randomly and recklessly without considering the negative implications of their irrational action on me. I would NEVER ever have anything to do with them or recommend them to anyone.