Breach of contract

I took a contract with Telkom, the contract was suppose to be for 20G anytime data and 20G night data. But all i got is 8G data. I went back to the Telkom shop to ask about this a week ago, I was told that IT is working on solving the problem but I have not received any feed back. I feel so disappointed by this poor service. At the end I will be expected to pay the agreed amount when I only got a fraction of what was promised to me.