Cell C do not understand the fundamentals of accounting

Cell C send me constant messages stating that I am due for an upgrade and I reluctantly decided to take the plunge, as I need a new phone (although purchasing a phone probably makes more sense, no lock-in). They asked if they could do a credit check (a bit offensive when you have been a customer for years, but I agreed). I passed the check, but for some obscure reason, Cell C were blocking the upgrade.

I had 2 contracts with them and migrated the one number in September-October 2020. I followed the process of changing from a contract to prepaid, and settled the outstanding balance according to the balance check details provided by themselves via USSD (the balance on this shows 0.00 owed, by the way). They claim that I have short-paid them and I have spent the last 2 months trying to obtain an invoice from them showing how I short-paid them and for what (the amount is in excess of the monthly contract amount, and the use on that account was minimal, as it was reserved for a friend of mine who died of cancer in March last year). Every time I request an invoice, they keep sending me a statement (sometimes for the wrong contract), with this bizarre koolaid comment "The statement is the invoice". I have just been interrupted in my work by someone to whom this problem was finally escalated, who again repeated that the "Summary Statement is the Invoice". The person I spoke to says she has no monthly accounts and does not receive invoices so, she does not understand my problem. Please note that no-one has ever contacted me to request payment on this disputed amount, it just appeared as a random reason for not upgrading my account. Apparently the query will be escalated further, but I will not hold my breath. They also have a very irritating habit of saying "No problem", which clearly is the case, or there would be none of these time-wasting phone calls.