Contract upgrade

I am a client of Cell C and have 3 contract lines with them. I have been a client going onto 5 years now. I have an upgrade due on one of my lines and I wanted to upgrade to a data contract that they are advertising ad nauseam where you pay R499 or so and get 60 gig and a free ZTE Router. I was told I could not upgrade to this deal as the contract is a voice contract and the deal a data one. What puzzles me is that the line I am trying to upgrade was initially a data contract which I upgraded to a voice line and this was not an issue. Now I am told I cannot go back and I am stuck with a voice contract I do not need. Possibly because they make higher revenues out of a product I do not need. Why is it possible to upgrade from data to voice and not the other way round?<br> Cell C is promising contract buy outs to people that I not even client of theirs yet as a client already you are restricted from moving from one contract to the other. So once you are in you are trapped.<br> I don't want another voice contract and if you are not interested in my business please let me know so I can move elsewhere and cancel my contract in the process. Telkom Mobile has a similar deal anyway