Account overcharge

Telkom has recently charged my parents account five times more than their contract. They say we activated this option but it is not possible because the registered sim card is always in a WiFi router they provided. The router has no other capability than to connect me to the Internet. Telkom is putting the blame on me and have taken a sum of R800 and more from my bank account. Thus, I have concluded that they did this intentionally, or a malfunction occurred and warn others from Telkoms way of doing business.