Account in arrears even after being paid up

I have paid up all my Vodacom accounts just under a year ago, and today I find out that Vodacom has not closed one of my accounts despite me providing proof of payment and a signed early cancellation form as per their request.

I also called the call center a few weeks after settling the accounts to confirm that the accounts were closed to which this was confirmed.

This is very frustrating as this will have a negative implication on my credit score for their negligence.