Outstanding Invoice after settled account

November 2020 i requested a settlement amount and was provided and settled it on the 11th November 2020. In December 2020 Vodacom deducted an amount and when i called they said it was an outstanding invoice for Nov. No one explained nor contacted me with this regard and today i received a letter of demand for something that was never communicated with me. That is just pure ripoff especially if Vodacom did not contact me. Why was i given an incorrect amount in the first place.