MTN Retentions

On 09/03/2020 I called the MTN Retentions department for my contract to be cancelled, I spoke to an agent named Thabang Letsoko who advised that he cancelled the contract only to find out a few weeks later that the contract wasn't cancelled so I called in to cancel the contract and it was cancelled and now I'm getting endless calls from Credit Collectors saying that I'm owing MTN an amount of about R6000, so I called the Legal department to find out what is happening and I was advised that Retentions department made a mistake and they have to credit my account and fix the error they made, I've been calling the retention and I'm not getting any help.