Incorrect billing for 6 months! No assistance

Cell C opened a contract with me 6 months ago and cannot get the billing correct. I have been billed incorrectly every single month. I phone on a monthly basis and send e-mails until I'm blue in the face and still they cannot get it correct. On requesting them to cancel my contract, they request a settlement for the full contract. If I cannot get the billing fixed for 6 months, how is this now my problem.