I'm not sure, but my ISP never complained that I am downloading or using the internet too much, and I never got a overage fee, so I think I never got over my bandwidth limit, or they don't care that I went over the data limit.
Got a fee in August of about $20... several times I have seen the 'you have used x amount of bandwidth this month and will be charged if if goes over x' pop up on my screen though.