Overcharged month after month

Telkom has charged me twice for extras that I have not asked for. They are very difficult to get hold of. When I eventually speak to them they say that they are investigating and I will be reimbursed but they don't know when. This is not good enough. They know when to debit my account but do not know that they are overcharging and when they will correct their mistake. Worse they are making the same mistake month after month.