This bad service is unmatched

I cancelled my contract in October 2020. I told the consultant that I do not use the same bank account anymore and I will be paying EFT. I received a "Final Invoice" from Telkom. I paid the full amount per their "Final Invoice" via EFT on 26 November 2020. On the 6th of December, I receive another "Final" invoice from Telkom. I was so angry because at this point I wanted nothing to do with Telkom and their terrible service. I spoke to their consultant who told me that they tried to debit my account for the final payment and it returned unpaid. But I had explicitly told Telkom that I do not use that bank account anymore. She was very rude and said I have to pay. I do not understand how a return debit order could be charged over R200, but the point is they shouldn't have debited my account.