I\M BEING ****** OF AIRTIME"

I loaded R110 of airtime yesterday and today it is finished. I made no calls in that time, I still have 300MB of data left. Then why do I have no airtime today. This is completely unacceptable, and I'm pretty sure that its been happening for a while now, because I keep on having to load more airtime on a regular basis, and I don't even make that many calls. This is completely unacceptable, you are robbing your customers blind!!!!!