INCORRECT BILLING

I have been an MTN Customer for nearly 20 years. I upgraded my contract last year in September. My average account is generally in the region of about R400.00 - R500.00 per month. As soon as I upgraded, my account suddenly shot up to R800.00 R900.00 per month. When I enquired, they acknowledged that they forgot to load my data bundle and as a result my account was much higher. They then corrected my account and gave me a refund, only to do exactly the same again the following months. During December 2014 they confirmed that they have a credit for me of about R1490.00. Yet when I enquired again in February 2015, not only did the credit suddenly became less, but they still keep billing me incorrectly. Chevonne at their Oslo Beach Offices tirelessly tried to have it sorted, but to no avail. I think I have been patient enough now. Furthermore, they also suddenly increased my second account with them, which is a top up account and should be exactly the same every month.