"Most people toss their phones after they're done; a lot of them give their old phones to family members or friends," said Miro Kazakoff, a researcher at Compete Inc. of Boston who follows mobile phone sales and trends. He said selling a used phone — which sometimes can fetch hundreds of dollars — is increasingly popular.

The 10 phones Trust Digital studied represented popular models from leading manufacturers. All the phones stored information on "flash" memory chips, the same technology found in digital cameras and some music players.

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Flash memory is inexpensive and durable. But it is slow to erase information in ways that make it impossible to recover. So manufacturers compensate with methods that erase data less completely but don't make a phone seem sluggish.

Phone manufacturers usually provide instructions for safely deleting a customer's information, but it's not always convenient or easy to find. Research in Motion Ltd. has built into newer Blackberry phones an easy-to-use wipe program.

Palm Inc., which makes the popular Treo phones, puts directions deep within its Web site for what it calls a "zero out reset." It involves holding down three buttons simultaneously while pressing a fourth tiny button on the back of the phone.

But it's so awkward to do that even Palm says it may take two people. A Palm executive, Joe Fabris, said the company made the process deliberately clumsy because it doesn't want customers accidentally erasing their information.

Trust Digital resurrected erased e-mails and other information from a used Treo phone provided by The Associated Press for a demonstration after it was reset and appeared empty. Once the phone was reset using Palm's awkward "zero-out" technique, no information could be recovered. The AP already used that technique to protect data on its reporters' phones.

"The tools are out there" for hackers and thieves to rummage through deleted data on used phones, Trust Digital's chief technology officer, Norm Laudermilch, said. "It definitely does not take a Ph.D."

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