Incorrect Billing of my Account for 4 months

My contract for data was was upgraded form 500MB to 1G which I was told telephonically was tol its gonna cost me less than R100 to my suprice when the billing statement arrives I was gonna be charged for R409.24 which when I enquired it was for 5G which I did not sign for it and it was debited from May same goes for June and July months. I have been calling the call centre to quiry this and firstly I was told about investigation which afterwards I got to someone elase who otld me I was refunded which it never appear in my statement its been as refunded I onmly get notification of debit but no credit credit from them its frustrating as all the time when I contact their office I get to have a new reference and I must wait for 7 days for feedback which always never get what I asking for to be billed correct for a package I signed for