vodacom contracts

I had a upgraded my phone at Sandton Vodacom. When I went to the store to cancel the my phone contract after 2 years they tell me that the quotation of the phone is 3 years. i request to the quotation document in question. Document stated the contract being 24 months but my account is still debited. i have called vodacom numerous times but none of them have helped me. How does one get to rectify this error?