MTN - Defamation of character and falsely accused me of non-payment

I visited the MTN store at Northgate Mall to upgrade a contract. After 1.5hours the agent screams out in front of other customers that my account was handed over for collection to their legal team. Yes, a truly embarrassing moment for me. To top it all off my account is paid in advance which I effect via EFT. The agent then said that I would need to call through to MTN on 135 and they would need to investigate. I called using the agents phone and the person who answered confirmed what the agent was advising me. I then asked the person to check what the balnce on my MTN account was. The person answered stating that it does reflect as a credit balance on the account, meaning that MTN owes me money. So for being a good paying customer, MTN now handed me over to their legal team for collections and the. Stops me from upgrading the contract. The agent on the line said that I need to contact their collections team during the week. Again, a frustrating hour odd trying to get a hold of the collections team. I finally managed to speak with someone called Nombhule in the personal collections team, only to be advised that she does not pick up anything on her side and that I need to call again at a more opportune time when someone is available on another team to investigate. I am completely livid by how MTN are treating me, a long-term paying customer. The ongoing inconvenience, the embarrassment, the ***** behaviour of a so called reputable business.