Mtn
Charged 12000 for data?

I am quiet disgusted and appauled with the service i have recieved from MTN.
I am being charged for Data usage of 6921.19mb but only used 1002.69mb how the is this possible?
Can someone please resolve this asap.
I dont think its fair for me to know and go dig up 12000 when i should be only responsible for a bill of R1754.70.