MTN airtime disappear

I'm a contract customer with MTN for the past 15/16 years. My current contract i receive minutes, data and SMS's. I am also able to load airtime (rand value). On 21 January 2020 I checked my balances around 6am and all was fine. Later the same day I noticed that the rand value was gone. I immediately contacted "my" service provider. After checking they finally said they will escalate the query and requested me to contact them in 72 hours. I waited and contacted them again. I was again informed that the query is still open with no result and should contact them again in 72 hours. In the meantime i bought airtime again. To my amazement the rand value disappeared again (i have no subscriptions). Again and again and again i contacted MTN with the same reply and various reference numbers. They can't tell me what is happening with my rand value airtime.