Urgent Situation- deeply frustrated!!

I took out a contract in 2013 with cellc, so before a contract ends or expires the company is supposed to contact the client if the contact has to be cancelled or upgraded right? In my case no one has contacted me or explained this to me. In 2018 I went to a cellc store to find out if I owe anything on this contract and I was told no funds are outstanding then i requested a paid up letter or settlement letter as proof as to what i was told according to what the system says. We are now in 2022, no one have emailed me anything ever since. The in june 2022 I have enquired and requested the paid up letter once again, instead they asked me security questions which i failed, due to me switching banks 5years ago... and the number that I am using is no longer the contract sim, it's a prepaid cellc sim. So everytime i phone in I'm being asked questions i cant answer correctly. They have my ID number to check what is happening on my account why cant they just give me answers on the settlement letter as promised years ago. And for a fine cheek they tell me I owe R807 on this same account 5 years later!!! I ask them a statement or anything to show me where this R807 is coming from, they cant give me anything as proof on my account owing R807!! I have been calling the customer care for months, emailed them as well but everytime it's a different person telling me the same flippin thing as to when I phone in, they dont provide me with banking details or anything just say that I owe R807. Today the 28th August I went in store again, the cellc store assistant checked on their system saying is paid up/prescribed meaning there are no funds owing but yet the R807 shows that they cant explain to me as well. The store assistant phoned in to customers care to enquire, he hands me the phone to speak and they ask me the same security questions, I answer as to the best I can, yet still I fail the security check, I got frustrated and ask them what is the hold up as they have my ID number she said she cannot assist me if I don't pass the security test. I then hand the phone over to the in store assistant. So basically it's a dead end with instore and with customer care ... I have been calling them for months with no progress, I am disgusted and I wont pay a single cent without a statement or any proof stating that I owe, I have emails and I have a reference number as to when I phone in. I am a fed up client really, as this still reflects badly on my credit score, and no one is able assist whatsoever. I need help asap!! CellC is the worst, that could be someone's else's debt that I am liable for now.