Vodacom agents ********* behavoir

I called Vodacom to cancel 3 contracts. In order to retain me as a client they offered discounted rates on the 3 contracts. I have been overcharged on these contracts since they were renewed and queried this numerous times. I received 3 calls from Vodacom in December, being the 11th, 17th & 23rd during which I had to explain the issue to each agent. On the call dated 23 December, the agent said that the calls on which the contracts were renewed can't be retrieved due to a technical issue. This is not my problem Vodacom !!!. I want the benefits and rates that was agreed to on the calls or the contracts will be "null in void" and you can cancel them and refund me the overcharges.