MTN ******ing money from customers after account have been paid in full

I had a cellphone contract account with MTN that I paid in full on the 29th August. I never received a paid up letter from MTN because they don't proactively communicate with customers. Today (20th September) I followed to confirm about the paid up and to my surprise I am told there is an additional R623 outstanding because my invoices run on the 13th and because I did not follow up before the 13th therefore an additional invoice has been generated and needs to be paid.

I find this to be extremely unfair as when I settled my account I was told there is no further amount outstanding. The running of invoices is an internal process that I as a customer is not privy to.

Can you urgently assist me as I feel the additional R623 amount is an ******ion.