Insane amount deducted from bank account

I am for almost 4 years a client with MTN and since this new \price increase"I'm starting to receive insane amounts of bills for the past two months. My account is always like R400 at the most but since July I started receiving bills of R1100 and R3150. Since I work from home and 90% of the day in a WIFI zone I would like someone explain to me how this could be possible? I barely use my phone and data for social media but I have to be billed with this amount and pay it? This is insane really! And the best of all is that since I've been using my iPhone I've been billed with BIS for almost 15 months but this be kept quiet because who knows? This is really poor service and I would really like some answers. I phoned the call centre and went in store but no one really cares about your huge amount it must just be paid. """