Impossible to cancel / go on prepaid at MTN

My MTN contract expired on 1 July 2020. I visited the MTN store at Cresta, Randburg to cancel/downgrade to prepaid. I was advised that my contract is now on prepaid. After following up by phoning the customer careline, I discovered my number was still on contract. I called the MTN careline and after being put on hold for 30 mins, I was promised my contract is now canceled and on prepaid. When following up on 15 July 2020, my number was still on contract - not canceled and I was still invoiced.