Breach of Contract

I was contacted by a call centre agent in May this year. I agreed to a data contract for 2GB's at R89 per month. I ensured that I capped my data at 2GB's so that once the 2GB's were used i would not go over this limit. My first debit order went off for over R600. I have been informed by MTN that my second debit order would be over R300. How does this make sense??? I have contacted MTN on numerous occasions for assistance, and spoken to more than 10 consultants! I have 7 reference numbers and those are only the ones i wrote down . NO ONE has assisted me in any way. Often they just drop the call or transfer to another person who does not assist either. I do not know what to do anymore! no one should agree to MTN contracts via telephone. The service is atrocious; in fact it is none existent!!