Mtn advert really misleading

Somewhere last week,I wrote a complaint about the treatment Mtn is giving me.As of now, no one has called or contacted me.Im still sitting with a bill of R1500 for a mere 20 gig which Mtn in their advert says it cost R299.I am not going to pay that amount, infact I am about to cancel that contract, if they still remain quiet or if they insist that I pay that amount.No one has come just to explain how they arrived at this amount or if the advert was incorrectly placed.