Over billing

Since July there has been an inconsistency of Telkom debiting my account. when I called in they told me Telkom owes me money they over debit and asked that i send through my bank statement of which I did. After a week I called to find out they told me they do not owe me anything. Last night they sent me an invoice which is triple the amount I pay normally and when I called them they told me its because last month I made a cash payment- which I have never made. They put me on hold for 30 minutes until I ran out of airtime. Terrible service. And I have to bare all the extra costs.