Cell C - I have seen for myself

I took up a data contract with Cell C in 2013, the contract was a voice recording with an agent working on the behalf of Cell C. In the voice contract I explicitly told the agent to cap the data at 2 G per month because the data was used by my son. In Feb 15 I received a bill of 2 400 and 2000 rand bill for Mar 15. When I got the first bill I queried the bill with Cell C and they claimed I had not given the agent instruction to cap the data line. In their investigation Cell C found the voice recording where I gave instruction to the agent to cap the data line. Having seen their mistake Cell C has come back to request that they split the bill 50:50 for their mistake. I not prepared to pay for the data usage beyond the 2 G for R99 we agreed in the voice contract, furthermore, Cell is reluctant to forward me the voice recording of the contract.