Over billing and poor customer service

I bought 15 minutes to make calls for 7days. On the 24 i made calls that took about 8 minutes and my minutes were elapsed. When i call customer care for assistance i spoke to a consultant by the name of Nompilo Mthana which couldn't assist me. I then asked to speak to her manager which i forgot his name he still did not assist me. Instead he claimed that his system was down and my line is breaking then he hangs up. Whenever i make calls i get over billed and they can't explain how.