incorrect billing

Last month I had a bill of R970. I called and was told I'd be sent a report showing why my bill was high. I'm still waiting. I received my invoice via email today and again I see that I'll be billed almost the same as last month. I haven't used this sim card at all this month and am almost to now at my wits end. Can someone from mtn explain how cell nr *** has a bill of R636 when the sim card has not been used? I also want answers wrt May's billing. I have enough to stress about and don't like it when I'm made to pay for expenses I know nothing about. This billing drama never happened when I was under Nashua. I want answers otherwise I'll reverse my debit orders.