[germancito]

I've compared the network usage as reported on the iphone with the one AT&T measures, and it's not the same. I've paid over-usage fees at times when according to the iphone reading I had 20 MB left. Now it all makes sense...

[sukuriant]

Where did it say you had 20MB left? Were you using ATT's evaluation? Or a third-part app? Or, the iPhone's operating system, itself.

If you're using the iPhone's operating system, ATT may be calculating space, and rounding up in a different way from Apple. (Examples of this exist in other child-comments of the OP)