Airtime getting finished without a reason!

Hi,<br> <br> I have a Blackberry handset,which I use regularly to browse the net.<br> I purchased a BIS bundle on the 13 November 2015,with a balance remaining in my account.<br> I attempted making a call a few hours later,but to my utter astonishment,my balance had been depleted.<br> I did not use my phone during the period for anything but browse the net,which should have fallen under my BIS bundle,so why was I charged?<br> This is definitely not the first time it has happened,and I am sick of this!<br> <br> Thanks