Excess Billing on my account

On the 25th of August 2015, I had an irregular debit from MTN. My handset contract including my data contract has a sum of approximately R650. However on the 25th, a total of nearly R1150 was debited from my bank account. After numerous calls with the MTN call centre, please help to resolve.<br> Ref: 133 429 355<br> Ref: ***<br> 1. Please explain why I have an outstanding amount<br> 2. How this amount began and when<br> 3. Why I was not informed of this amount<br> 4. Why the debit was made in August 2015<br> 5. What I did to get this amount in the first place<br>