Overcharged for 4 consecutive months and this month the overcharge increased. Permanent solution must be found.

Accelerit has been my internet service provider for just over a year and all was well until April when the invoice was overcharged with R93. I called in, they said it was system error, apologized and said they would credit me back what was overcharged. They did credit me but May, June invoices were overcharged with the same amount and resolved after I called in in both months. NOW July invoice is overcharged with R200. I called in again this morning, they apologized and corrected BUT i requested to speak to manager because I cannot keep on calling them every month(wasted of my time and airtime) for the same thing AND the manager HUNG UP on me. 1. Can Accelerit fix their billing system permanently. 2. Can their accounts department take user queries seriously and actually make sure queries are resolvedddd. Otherwise, I might claim for my airtime from them now.