Complaint on Bad Service = ***

In July Telkom ran a special of 20GB day and 20GB Night then an extra 20GB anytime data. I took up that contract but they then split the extra 20GB into 10GB. Instead of the full 20G anytime. I have made numerous calls and everyone I call refers me to someone else. How do I get this fixed and cancel this contract which by the way will be my last with Telkom as this happened before