Not honouring agreement

Vodacom Retention call centre made me a counter offer to stay with Vodacom. I accepted the offer, received my phone but when I verified the plan, I found that I received the standard plan and not the offered plan. Vodacom now claims that the voice recording cannot be located and I am stuck with a two year contract that I did not agree to. I am phoning almost daily and it is not being resolved. Please help.