Incorrect billing by Multichoice

I have a price lock with multichoice but for the past few months I have been billed more than R150 more and when I call in I am sent from pillar to post. All I need is for multichoice to explain why they are debiting me R1 050 instead of R800. For one call I spent 54 mins listening to music without any help. Today the lady I spoke to said her system was slow and also played music for more than 17 minutes until I dropped the call as I had run out of airtime. She didn't bother to call me back. Their service is pathetic.