The "Daily Mail" reports a team of engineers did an investigation and found the telecom giant was overcharging even when iPhone users weren't using their phones to download data or surf the Internet.
A newly filed lawsuit alleges customers were routinely overcharged between seven and 14-percent and sometimes as much as 300-percent.
An independent probe confirmed the engineering team's allegations of overcharging.
It happened even when all software to access the Internet or download data was disabled on several iPhones.
AT&T says some apps have software running in the background that can use data.
Until a few months ago, AT&T was the only network for iPhones.