More than one in four U.S. wireless customers had unexpected charges on their wireless phone bills, and many don't know where to lodge complaints, showing the need for better industry oversight, congressional investigators said.

About 10 percent of wireless phone users are somewhat or very dissatisfied, the Government Accountability Office said in a report released yesterday. Almost a fifth wanted to switch services but didn't do so, with many citing cancellation fees as the reason for staying with their carriers, the GAO said.

While most users are satisfied with their service, the survey indicates the Federal Communications Commission should strengthen efforts to help mobile-phone customers, the GAO said. Areas of concern include complex billing statements, termination fees, unclear explanations of service, dropped or blocked calls and long waits for customer service, according to the report.

The report "suggests that millions of consumers still experience problems related to billing, fees and customer service," California Democrat Rep. Henry Waxman, chairman of the Energy and Commerce Committee, said in an e-mailed statement. "It is critical that the FCC, Congress and the wireless industry monitor this situation closely."

Last week, the FCC asked Verizon Wireless to explain why it doubled to $350 the fee customers pay for terminating smart-phone contracts, and four U.S. senators introduced legislation to limit the fees.

About 84 percent of adult wireless users are very or somewhat satisfied with their wireless service, said the GAO, the investigative agency of Congress.

"While the percentages of dissatisfied users appear to be small, they represent millions of people," the GAO said, citing estimates that there are more than 189 million adult U.S. wireless users.

The FCC has conducted "few in-depth analyses" of customer complaints, "impeding its ability to determine whether its rules have been violated or if new rules may be needed," said the GAO.

The FCC said it is improving its complaint tracking and has "begun work to address many of the very concerns raised" in the GAO report. The FCC's comment was in a letter signed by managing director Steven VanRoekel that was appended to the GAO report.

More than 85 percent of U.S. wireless customers are served by one of four major carriers: AT&T, Verizon Wireless, Sprint Nextel and Deutsche Telekom AG's T-Mobile unit, the GAO said.