Telkom billing system taking money after contract has been cancelled

Signed a Telkom home data contract which was on special (R374.33 per month) on 29 December 2017. The first amount that was deducted via debit order was on 25 January 2018 was R431.81 which I was told was the pro-rata amount for the 2 days in December 2017 and the charge for the month of January. Fast forward Nov 2019, I was told to submit a cancellation request on 29th Nov, exactly a month before the contract expires so that it will end on 29 Dec. However a bill was sent to me 25 jan and when I enquired I was told this is for the month of December because the billing is done in arrears. If the billing is done in arrears why did I get charged R431.81 instead of only the pro-rata amount for the 2 days in Dec 2017?. This billing is not right and no one wants to admit it. I have gone to the call centre in Fourways mall, spoke to Zakes Mandiazi who referred me to billing call centre, who hung up the phone after explaining everything and said they were checking. I then asked for the call centre again then Zama gave me her email address *** to send the first invoice but she never got back to me. I had to reverse this debit order for Jan 2020 from my bank account as no one seems to be helping me resolve this and I don't know what else to do as another bill has come for end of February and the cycle will continue.