Settlement amount paid

I am still trying to make sense out of what happened with my contract with Cellc. I have a mobile contract that expires in Jan 2022. In July I made a full payment to settle my contract which by the way I called in first to request a settlent amount and not closing the account. Payment was done and proof of payment was emailed to ***. Few months went passed without being debited as it should be. On the 20th of November I was debited R339.75 (less than my usual installment). I called customer service to dispute the amount and to my surprised I was told it's for my remaining balance. I asked them how could that be possible because according to my knowledge I had paid the remaining balance in July already. I understand the fact that I had not requested to close the account because I wanted to upgrade and continue with the contract. The lady who was assisting me kept on emphasizing that it's for the remaining months till the contract expires. I need to get a proper explanation as to what does the settlement amount covers and why am I still owing on this account. I am really not happy with how cellc can decide on just debiting my account after I had made a full settlement amount. I want my money reversed back into my account and close this contract because I cannot be paying for hidden costs. I want this issue resolved before I take this further to Ombudsman.