R9000 billed without explanation

So to start, I have been with Mtn for more than 4 years and 2 times now my father has had to pay bills of R2500 and R6000 both without explanation. This December we were up for an upgrade and while doing the upgrade we change the usage limit for out of bundle charges to R0 after the other 2 bills happened. So after the upgrade I received the 30 GB FREE LTE DATA that was part of the December upgrade promotion and started to use it. <br> <br> Fast forward to middle January and I notice that I still have alot of Data unused and that it will be deactivated a few days later. Thus I start to use it. I continually checked my data balance until the date that it expired and when the expiration date was met to see that I had about 1-2 GB unused. <br> <br> After that I continued to use the other data bundles that I had left until today when I checked my invoice for this month to see that I owe Mtn R8000+ for data. When I called customer support they had put the phone down in my ear. <br> <br> I would please like an explanation off how I got billed on FREE data.