Not understanding where your airtime goes to

I have been using MTN since 2005 and I have been happy with their call rates but since the middle of this year (I don't have specific dates), I have been experiencing problems. I use a Blackberry Bold so I most usually subscribe to BIS uncapped. To my understanding, this enables me to use social services and the web without having to put in airtime. The trend that I have noticed is that every time I recharge, airtime is being taken away and I don't understand why because sometimes I notice that my airtime finishes without even making phone calls. The last time I recharged, I used R10 airtime and I only made two short (seconds for one and 1 minute for the other) phone calls and I didn't have airtime after that.<br> I would like to get an understanding of what are we paying for and what does BIS uncapped mean.<br> This issue had me considering changing to another network and I am not the only one with this problem, a few friends who are using MTN are facing similar problems and are also planning to change networks.<br>