Breach of contract

I was offered a contract with mtn for R99 a month for 12GB.it was an over the phone agreement.To my suprise mtn is debitting more then twice the amount.I have tried emailing but no reply.i then discovered that the two cards i was given was the reason i am paying more than what we verbally agreed upon.i am now now wandering am i the only one with the same query or is this how mtn operates.Totally dissatisfied.