Billed for something I did not sign for or agree!!

My contract recently expired, I have been to the N1 City and Canal Walk branch on more than one occasion to find out if I should upgrade or if I can keep my existing contract as there is nothing I want to upgrade to at this stage. I was advised that I can keep my existing contract on a month to month basis and the installment will remain the same and that I won't be charged for the handset, the manager at the N1 City branch confirmed this! My monthly installment is usually R369. However, I received a bill of R788.39 as the conditions of the contract has changed without my knowledge or consent! This is against the Consumer Protection Act and Unfair Business Practice , you cannot just bill someone for something that they didn't sign for or agree on !! I want this bill rectified and contract cancelled with immediate effect, I am not interested in any stories.