99c per MB charges and Unauthorized limit change

Twice I requested new services on the MTN app and twice I have been told that my limit has been changed. Why does MTN's system change my limit automatically without my consent? At 1st I was blamed and told that I requested the limit change using a USSD number that I don't even know. That change of limit also meant that there was a change on data charges (99cents per MB) which is madness. So far MTN has changed me +-R1600 for about 1.6Gig of data. I have contacted the billing department and the reasons they are giving me don't make sense. I feel hard done by especially as I have been a loyal MTN client for more than 14 years. The service and lack of knowledge of what is happening with your systems lives much to be desired. This is **************** and passing on the buck to me is very unfair. I have always bought my own data when there is a need and have an 8gig I get from Axxess every month. I need this issue to be escalated as it looks like some people don't understand where I am coming from. Maybe its time to look at the competition