Worst Customer Care Service

I am not happy I changed my method of payment into cash last year in December I made a payment but mtn still debited my account so I told myself ok maybe it wasn't approved. So mtn continued to debited me From January till May all of a sudden in June the system decides to pick up that I am a cash payer. Why didn't anyone call to inform me that I need to pay cash for July I am highly disappointed in MTN