Data that disappears and not only now it happened in the past also

This is the 3rd month that my data disappears and Vodacom blame me for it when I know I have barely used my data. Yesterday it happened again but this time the data literly ran out in front of my eyes from over 500 meg to nearly zero into next no time. Before I could convert airtime into data the data ran into my airtime and nearly maxed out my airtime, fortunately I switched of all data otherwise I don't know what would have happened. Then I several different messages from Vodacom about the status of my data, the last one saying I have more than the first one I received, very contradictory. I did not use anything out of the ordinary. I tried to phone them 64 times and this morning again. Nobody answers the phone. It rings and just gets disconnected time and again. This morning the phone just rang for nearly 40min. Unfortunately I had to leave for work. I am powerless, I can't do anything and you can't get any help of Vodacom. They always have a very clever response and make you responsible for what is wrong on their side.