Incorrect Billing

Statement on the 7th of April was incorrect. MTN had asked me to pay and amount of R1349.78 which did not make sense to me as my contract was with Nashua mobile which was 200 anytime contract, 200 talk time a month, 200MB data a month, 200 sms a month and 30 min MTN to MTN a month. 27th of March I received a sms that I went R400 over my limit and that my phone has been locked. Not understanding what went wrong, I phoned customer service. After holding on for 20 min my call was answered and I explained what happened. The customer service told me that I need to pay an amount of over R1300 and I said I refuse to pay this amount. Customer service said that it does not show on my contract that I need to receive the free data a month of 200MB and no stop is shown on the R1500. I was told to contact customer service again on the 2nd of April so that they could investigate.I have already sent 10 or more emails with no response. I am disgusted with MTN that a month later no one has bothered to respond or give me some sort of feedback. Where is the customer service??? MTN can view my track record that I have never went over my limit so how come now? Please can this be sorted