MTN *****ING DATA AGAIN AGAIN AGAIN

upgraded my new contract with mtn. I took the 80 GB (50GB day and 30 GB night for R299.The new datawas loaded I only received 28.33 GB day and 37.05 GB night. now this is not the first time mtn is *****ing data for the customers. I had this problem before with one of my other contracts if I knew this was going to happen again I would have rather gone to Vodacom. Can somebody please sort this problem out because customer service and sales persons don't have any idea what they are doing. I wont be paying this bill till it is sorted out. The contract number ***