If you are with 3, do you know how to find out your Megabyte usage?
You go to the 3 home page and login
After you login there is a page what will have your mobile number on it.
If you click on that number it will send you to a page where it tells you how much you have downloaded since the month started.
If you write this down, and then go back there in 24 hours, you can subtract the new Megabyte from the previous and you know how much you have REALLY downloaded.

But the best way is if you have Windows XP, there is a Icon on the bottom left of the screen when you hook up to the Internet Explorer.
If you right click on it and then click on status, you can watch exactly how much Megabytes are being uploaded and downloaded in REAL Time and are the ACTUAL numbers, rather than that VOLUME thingo.