Cancellation of Cell C Contract

On the 09/11/202 I cancelled my son's cellphone contract with "Cell C" as he is in the UK and has not used it in over a year. After several conversations with Eric and then Namdiso, I was told the contract was cancelled but because my billing is on the last day of each month I would not receive my money back for this month and would only have to pay 9 days in December 2020 (hence the 30 days notice period). I had no problem with that, it is normal business practice. However, on the 30/12/202 I noticed my account had again be debited for Cell C and not the 9 days but the full month. I stopped the payment and contacted Cell C. I spoke to a lady called Eunice that said that this was correct as my billing date was the beginning of the month. I explained the above to her and she was adamant that I still owed a full month and pointed out to me that if I did not pay I would be handed over for collection. I asked her to send me an account via email (as I had not received an account from Cell C in about a year and a half in any format), with the correct amount on it and I would pay it by EFT, she sent me 2 invoices but again for the full amount. In these times when money is tight and business are trying to survive is this the way to do business? The thirty days notice was paid, if you include the 9 days I still have to pay, why must I pay for a full month and wherein the contract does it say that the 30 days is from the 1st of the month?