Disappearing airtime from my prepaid phone

My airtime disappears from my phone. I have phoned Vodacom customer care several times and the problem is recurring. After each call, I am not given an opportunity to rate Vodacom as a company, but the consultant who is assisting me at that time. How does Vodacom allow other companies to deduct airtime from my phone without me getting involved. Why is it not happening when I am using a sim-cards for competitors? I have been with Vodacom for years and I must say I am very unhappy with the service. This also happened to my wife. Imagine the number of people out there who are unaware and having the same problem, especially those who cannot read :-(<br>