Incorrect Billing

In February 2018, I called Vodacom to cancel my contract and convert it to a prepaid. They convinced me that I could get a contract package with them where I could get R110 worth of airtime, IGB of data and BIS for R79 per month. I then accepted this offer. Currently this is not what I am being charged, in the statement for April I was charged R207.82, in the current statement for May I am being charged R148.82 (my package does not allow me to use more than what is allocated so my costs can never exceed the package costs) . I have phoned the call centre numerous times, they cannot give me any answers, they keep on telling me that calls are being logged but my issue is still not resolved.