MTNs **************** billing

I upgraded my data contract on the 22nd September 2015, I received my new wifi modem but the SIM card remained. The data I received was not increased , however I discover that my bill was -+ R1400 for an account that's supposed to be R70. When I called the customer care , I'm told it was due to the fact that I had not set my credit limit to 0. This was first concerning because the SIM card was the same and the data I requested was the same and my credit was sat with the previous contract. Secondly when I called to set the credit limit , I was informed that the credit was set on the 24th of September by MTN. Why does mtn wait two whole days to set the credit limit , yet charge the customers for something they know to do without my request. Having a contract with vodacom and having upgraded with them around the same time, they inform the customers about the conditions of upgrading And only implement the upgrade once they have taken care of all the administrative logistics of transferring . This is to protect their customers from ridiculous charges that MTN imposes on us. MTN does not inform us at all that the line wiill be 'open' until they find time set the credit limit, which is c