Billed extra on top up

When i took up my contract in March i was supposed to pay R499 on a top up package. For the past 2 months I've been charged more than R700. I've tried calling the customer care number but i would hold for a long period with no one picking up. I went to Clearwater MTN where i signed up and told them my story and when the lady looked she told me i was billed out of bundle which doesn't make sense to me because I always buy airtime and convert it to bundles whenever the 500MB expires. She logged a query for me and 2 weeks later i haven't received any communication from MTN. I'm really regretting the decision to change to MTN. I'm spending a lot of money already on airtime for me to be charged extra. The statement i receive is even vague in itself. If you are going to tell me you've charged me more, i need to know when and why. It doesn't make sense for me to just see an amount just appearing on my statement. I'm even considering paying up this stupid contract and porting out of MTN. I need answers please, i cant be watching my money disappear mysteriously and not say anything. I buy more than R200 extra airtime each month already.