Overcharged on cellphone contract

I am beyond irritated at the moment. I have been overcharged on my cellphone contract since October 2015 and it has yet to be resolved. I have queried it on 24 November, 30 November, 14 December, 30 December, 20 January, 10 February, 17 February, 18 February, 20 February. Most times I don't even get a reply. I have never exceeded my included minutes and data on my contract yet they have been charging me R 150.00 more than stipulated on my contract. At first they would not recognize the error, but on 30 December I received a mail admitting that an error was made and that it would be resolved. On 20 January, when I queried if it had been resolved, I was told that I had to wait 21 days for a refund. On 10 February, I again queried when the refund would be processed, I was told that they would check with the credit department. On 17 February, when I again mailed them to find out what was happening, I was told that the amount that I was charged was correct even though they have charged me an extra R 150.00 per month for 4 months and after telling me that a refund was due to me. Mails that I sent on 18 and 20 February have not been answered.