Do not deliver on requests

We have been billed numerously for exceeding our data limit on the tables bought for the kids. In excess of R3000 pm. I contacted the Call Center and the lady claimed they cannot cap the limit on the contract. We should set the limit on the tablet. We were billed again and when I called again, the other agent said we can migrate the package to a top-up package. This was not disclosed in the original conversation. We subsequently purchased a data bundle extra for the tablet. This month we are billed again for extra usage. We again contacted the CC and now it turns out there is a system problem, they cannot add a data bundle to the tablet, which was not said in the previous conversation. We had purchased the bundle and reset the limits on the tablet, but now we are billed again as the agent couldn't load the bundle.