It's been called the biggest crackdown on telephone fraud in history -- an eight-month nationwide sting that culminated last week in the arrest of 10 suspected con artists.

The men are accused of cheating more than 3,500 investors out of about $50 million by selling bogus oil and gas investments. Victims have been found in all 50 states.

To make the arrests, officials raided 32 telemarketing firms the accused con artists operated in Los Angeles, Dallas and Salt Lake City.

The North American Securities Administrators Association, a group of state securities regulators, organized the sting.

In recent years, telemarketing scams have become a major problem for state and federal regulators -- with consumers losing an estimated $10 billion a year.

"I want to make it clear that we are not declaring victory on telemarketing fraud in the United States," said John R. Perkins, Missouri Securities Commissioner and NASAA president.

"Many of the con artists . . . will pop up somewhere else," he said. "It's like the problem with cockroaches: You flip on the lights in the kitchen and they scramble out of sight, but they're still around somewhere."

Investors who lost money in the current cases were called by sales agents promising quick riches from oil and gas investments in the United States. They were told the Persian Gulf war would drive up the price of energy. But in fact, officials said, none of the telemarketing companies owned any oil or gas supplies.

Some victims invested as much as $30,000.

NASAA members said they expect perhaps up to 30 more arrests.