One in six cellphone users has been hit with an unexpected bill that didn't reflect their monthly service plan, according to a report produced by the Federal Communications Commission.
The report also found that half of cellphone users and two-thirds of broadband users weren't aware of early contract cancellation fees.
"Among those who stated affirmatively that they would incur a fee if they tried to terminate service, many did not know what the fee would be," it says.
The report is a first stage in an investigation into 'bill shock' promised by the FCC last month - and extrapolating from the results indicates that as many as 30 million Americans have experienced this at some time.
Just a few days ago, a Massachusetts man was let off by Verizon after mistakenly running up a $18,000 bill.
"As we know from our consumer complaint center, even an unexpected charge of $20 or $30 can make a difference to many people," said Joel Gurin, Chief of the FCC’s Consumer and Governmental Affairs Bureau.
"Several carriers are taking steps to make their fees and billing more transparent, and we would like this to become a universal practice."
Many of the bill increases were comparatively high, with 23 percent reporting that they'd been hit by a rise of more than $100. And 88 percent said their cellphone company didn't contact them about the increase, even after it had taken place.
"The wireless industry has achieved remarkable innovation -- and mobile is increasingly essential to the daily lives of Americans," said FCC Chairman Julius Genachowski.
"But there is still more that can be done to help customers navigate what is sometimes a confusing marketplace. A simple and easy to understand mobile purchase and billing process will empower consumers to avoid bill shock and other unexpected fees."