Billing department ERRORS result in blacklisting without clients knowledge

My credit record shows I still owe Cell C on an account that I requested the billing department to merge my 2 contracts into 1 account. I learnt this when I received a report from the credit bureau stating that I owe 525 on a non-existent account. Cell c has been getting payments since last year but because of incompetence my credit record in the drain. They have received over R5000 towards the merged account because I was under the assumption that this was the only account I should pay as both numbers were appearing on the Dec statement onwards. It's now June 2020 I have requested that this error be rectified to reflect correctly on my credit record as I will not accept a situation where an organisation has been benefitting from my support and yet they tarnish my credit record.