Incorrectly billed for international calls

MTN billed me for international calls that cost R1154.78 that I never made. This is from my Samsung tablet which I don't use for calls. I have never made calls or used data on my tablet, I can prove this in my call history, and my monthly statements have been the same since inception. I only ever connect via WiFi. I never authorised international calling. A representative told me that international calling was activated on my account during a \special"on the 6th of July 2015. In September I'm getting billed. I cannot call MTN as when asked for my MTN number the robot voice says it's not a valid number. I'm afraid of receiving more bills like this in future"