airtime just depletes without me using it!!

I am so disgusted with Cell C. This has been happening for months now and not only to me. Someone needs to do something about this and quick. We can never leave any airtime on the fone. It just depletes, most times overnight it disappears off the fone. And I know I never use it. On my vodacom line, my airtime balance can stay for weeks. I keep airtime for emergencies, because I'm home mostly, and have an unlimited landline. I cancelled my Cell C contract for this same reason, they were overcharging me. What to do? I need my number. Or I would cancel this line. I invite others to also tell us if this has happened to them. Already most of my friends and family have the same complaint. And I can give their numbers if needed. Please can someone sort this urgently. Thanks for this amazing platform.