Airtime

Last night I loaded airtime and my balance was in excess of R90 and I made calls, 10 to be exact which don't add up to 28mins and now I am being told I have less than R3 remaining... how can this be right? This isn't the first time this is happening, my average spend on airtime is now an average of R55 a day... if u do the math thats R1650 a month on just airtime, lest we not forget the data that just miraculous disappears 1GB costs R149 and I average 1GB a week now thats R600 a month and collectively this is in excess of R2K a month. I need answers coz this is now becoming unacceptable from Vodacom.