BILLED FOR A CONTRACT I DID NOT SIGN UP FOR

I have submit an complain several times also via email, they say they will contact you with in 24 - 48h not so much, I am with Cell C for more then 10 yrs not sure what is going on, Some one phoned me with a package because i can up grade earlier, not the the futures she said it will be, I have send the phone back to Cell C, and all of a sudden i received a bill R1300.00 for Sept and now R1180.00 for Oct , apparently i signed up for a 800 NOT SO MUCH, I see on my \my account on line that they have changed it to the 400 package again but neglect to credit my account