Over-billing and poor customer service

After receiving a data bill for R862.05 , my usual bill is R129 I tried to call customer services at MTN to resolve the issue. Each time I call no matter the time of day the message comes up that they are experiencing an unusually high volume of calls. I assume it is not unusually high if it is always like that but rather they have not enough staff to efficiently handle calls. Each time I got frustrated and eventually gave up as I could not get through. Today I after much perseverance I got through and relayed my issue. The customer service representative could not help me and said she would escalate it to the billing department. I inquired what that process was and she said they would get back to me in 7 days. I then said as I have no faith in that happening and I also require my money back please transfer me to the billing department. She then said they are not taking calls. I noted that it is a call center and I want to speak to them she declined and then I asked to speak to her supervisor after which I was cut off. I really just now ant my money back and to cancel the service that I a very unhappy with.