MTN continues to dissapoint

Within the first week of moving to MTN, after being with another provider for a few years, I encounter the same type of issues I had when I was with MTN before. And, the same poor service when I raise an issue with them. Migrated to MTN about a week ago. Last week, ~26th April, I raised an issue that my airtime was depleting faster than I was using it. I'd purchased 180min voice bundle. After 82mins was used only 72mins remained - already 26min unaccounted for. They brushed me off, saying I should wait for the system to update. I waited over the weekend. The issue got worse. After the weekend, there was only 23mins left of a 180min voice bundle despite there only being <120mins used as per the itemised billing. ~40mins airtime can't be accounted for. I've raised a case with them on 3rd May. They have yet to response at all. I have called them every day since and they are still unable to give me any feedback, not even confirm that they agree something is wrong. Notwithstanding that I only got through to an agent on my tenth call today. And, the fact that I have already been billed twice for the same call.