Billed in excess of Credit Limit as set by MTN

I have been trying to communicate with MTN regarding a bill for data usage since June of this year. The outstanding amount on my account was R 8 200 for data usage. I was not informed / warned / notified in any manner and the argument is that the credit limit allocated to me by MTN is R 3 900 - this even appears on my monthly invoice. I received sms notification on Friday stating that the services will be suspended unless I pay the outstanding amount of R 8 200. It has been a complete nightmare as I have had to follow up via e-mail because the call centres just play music in my ears for up to an hour at a time. I tried to contact the legal department on Friday to discuss payment arrangements, tried for over 2hrs and could not get through. Overall the service has been extremely poor with NO feedback given only notification for suspension of service. Strangely enough the response was quick enough after payment was made on Friday. Although payment has been made I do NOT regard the matter as being resolved. I will be seeking legal advice with regards to my rights as a client as well as the Consumer Protection Act. Why does MTN allocate credit limits if they do not serve any purpose?