Unhappy and taken for a ride!!!

My cellphone is due for an upgrade, got a call from a sales agent and was sold a new contract with him promising me that I would be able to keep my number once the current contract comes to a end which was the 5 October, Went into mtn to have my sim swop done only to be told that I wont be able to have the new contract moved to my old number. I was advised to call mtn as I have 7 days to cancel the new contract which I did. The lady advised me that they will listen to the recordings and get back to me in a day or two. A week went by and then we called mtn again as nothing has happened only to get told it will cost us R6000 to cancel the new contract! Not happy at all and will definitely not be using mtn again after this!!!