Paying for Service? What service?

Shortly after that I recieved a SMS stating that I have exceeded my credit limit of over R1950.00? My credit limit is set to zero, once my data and airtime is spent, it's spent. How could I possibly spend almost 2K on airtime, 5 minutes after my account was deactivated and if my data was only on for Whatsapp? (iPhone) It doesn't make sense. Now I can't make phonecalls, send sms's or use my data. (Which I still had a lot of before my account was deactivated.)

Seriously MTN this is the second time this has happened, this month only. Previously I requested that you reactivate my account. Which was the beginning of this month (April, 2018) and it took almost 5 working days before anything happened and when it finally came on I had LESS than half of my user bundle for the month? Most of the customer service staff are careless and deliver poor service. I spend almost half an hour on hold to have a 5 minute conversation which leaves me unsatisfied and frustrated.