wrong billing on account

In November 2014, we signed up for your Simple plus offer of R 499 per month. It is now 5 months later with numerous visits to Telkom in the Woodlands shopping centre because I get billed way more than the R 499 as per your offer. Apart from the rude service, they are still unable to fix this and then Telkom dares to charge me interest because I refuse to pay anything more than what was agreed. Please fix this asap or have my contract cancelled with immediate effect. I am sick and tired of Telkom.