Credit Limit changed, no notice given, no explanation, have to now buy airtime even though I'm on a high end Contract!

I have been a customer for over 20 years with MTN, and had a credit limit of about R5000.00. I have 2 other contracts I purchased about 4 years ago, with a credit limit of R3000.00 each. Last month, while on a call, I got a message saying I have 1 min remaining on my pre-paid airtime, and my call was cut off!! When I phoned MTN, they said I had exceeded my Credit Limit, and had to buy more airtime. I told them it was impossible to have exceeded the limit. She then informed me my credit limit is R129.00!!!!! I asked her why it was changed, she said it was based on my salary, and every time I upgrade to a higher contact my limit decreases. Firstly they don't know what my current salary is ( and that is much higher than when I had the R5000.00 credit limit) , plus they let me open the 2 new contacts ( even though, according to their credit limit, I shouldn't be able to afford it???) Secondly I have never missed a payment in the 20 years. I was told she would send the query through to be rectified, but I have to buy more airtime in the interim. I have just run out of airtime again, and forced to pay another R150.00 ! This a total **** as I can't understand why I have so much data left, even though I've allegedly reached my credit limit. Needless to say I've also had no response, or any effort to keep me as a customer. Luckily my contracts expire in January 2018.