Telkom overcharging since we signed the contract

Last year we signed up for Telkom's 10mbps line with 50gb cap for R699. This included the line rental, isp, and adsl rental. We do not have a telephone set connected in the house and we have not exceeded the monthly cap once. Telkom has however charged us more than the R699 every month since we signed up. I logged tickets in the Tygervalley branch to have them look into this and never received an answer. I have contacted them on Twitter and the only response I receive is that they have referred my query to the accounts team, or they have escalated it with the accounts team. The latest round of me asking what is going on started on 12/01/2016 and I am still waiting for them to answer me about why they are taking more money than they said they would on the contract.