Upgrade department not honouring their verbal contract

I upgraded my cell phone contract in the first quarter of 2018 and the Upgrades department (telephonically) offered me the phone I wanted with the data bundle I wanted (modified to my requirement at no extra cost for 24 months), which I accepted. 6 months later I am getting half of the data I was promised per month and when I call to try and sort this out they state that that is the contract I signed up for. It is not and I have no recourse but to see if Vodacom will respond if I follow this route. So disappointed.