Vodacom incompetence

I had a contract which expired in December 2020. I called Vodacom and asked to cancel the contract and go onto Prepaid. To my surprise i got an invoice in January 2021 showing that it was not moved to prepaid. I called 5 consultants, 5!!! Each one telling me they will make sure it is cancelled. I received an email today (13/02/2021) and it has still not been moved to prepaid. I cant talk to these people anymore as nobody does what they are supposed to.