airtime

i have experienced this not once not twice but a few times now and my wife's phone the same problems .<br> our airtime and data just disappears for no valid reasons and if we phone thay just tell us Internet usage . but how can i go a whole month with 300 MB data and then one day use 200 mb is five min .<br> and no updates, update with data , wifi only . and they can never tell me why and how ..