Vodacom overcharging

At Vodacom can you please explain what is happening to our airtime every time I load airtime a day later it has decreased by R5 or more without me making phone calls. What is the point of buying data and all this other products if you still charging us from our airtime. Yesterday another tipical example I bought airtime I didn't make any phone calls but this morning my airtime is less by R5. It bad enough the prices ur charging us are ridiculous and don't last, most of us working people are at work 8 to 9 hours in a day later when we get to office our work WiFi automatically pick's up and our data is off the whole day so question is what is then decreasing our data if they are off. Stop making money out of honest working people who work hard for their income. Vodacom is not even loyal to their long standing customers they just don't care.