New contract not committed to and poor customer service

Multichoice(Isaac) phoned me several times in August to let me know that I have been rewarded for being a loyal customer over the years with a special package. I accepted on 24 August 2018. My premium would be R789 per month and additional R90 for Xtra View(Total R879). Once installed it would be for 2 years on debit order. They have been charging me for the normal R899 since installation. I have made several calls to the customer care centre ref SD ********** 4(one reference) to correct this to no avail. Every agent thus far argues with me with regards to the offer I received, do they not record the conversations? I confirmed the offer several times and I know it is correct. I absolutely regret signing up for 2 years - I should have known better than being duped. How can they sell you something and nobody seems to know about it