If anyone telephones with the glad tidings that you've just won a free vacation, hang up. You're about to become a victim of a telemarketing fraud that started in Southern California and is now reaching out to all corners of the country.

The objective of the fraud is to get your credit-card number: Visa, MasterCard or American Express. Unauthorized charges will start showing up promptly on your credit-card bills. "This scheme is growing in leaps and bounds," says Guy Wirsig of the Los Angeles Better Business Bureau. "It's just out of control."

Incautious customers take the bait because it's so juicy. A good example of how these travel-club scams work can be read in a complaint filed last month by the California attorney general against the Executive Gold Card Travel Club.

(Herschel Elkins, an assistant attorney general in that office, believes that this particular club is now out of business; no one answers the telephone number I have for the club.)

Salesmen for Executive Gold Card, and others like them, work out of low-rent offices with banks of telephones known as "boiler rooms," calling people around the country with a standard sales pitch, Elkins says. They imply that they're connected with one of the major credit-card organizations, and say that a computer has selected you for a free vacation in a fancy spot like Hawaii.

The "only requirement" for joining the travel club and getting your vacation, they say, "is that your present credit card be current." And that's the start of the slippery slope.

First they soften you up by giving you businesslike details about their own organization -- name, address, phone number, all apparently on the up-and-up. Then they ask for details in return: Your credit-card number and the expiration date. After a bit more patter, they'll also ask that you "verify" your address, the spelling of your name and one more deadly little question: "That's the way your name appears on the credit card, isn't it?"

If you give all those answers, you've effectively given your credit card away.

As an added inducement to talk, the salesmen claim that travel-club members get a huge cornucopia of benefits: discount buying services, cars for 70 percent off retail price, hotel rooms for 50 percent off, and a free TV or computer as a sign-up gift. A membership costs anywhere from $298 to $399 -- but the salesmen swear on the graves of their sainted mothers that no charge will be put on your credit card until you get the literature and agree to go ahead.

What happens as soon as you hang up the phone? You guessed it. The "membership" charge is put on your card right away (although you won't see it until you get your credit-card statement the following month). Other unauthorized charges may also be made in your name. The promised free gifts and vacations fail to materialize.

There are variations on the scheme. One travel club gets your credit-card number by offering "lower prices than you can get from any airline or travel agency" -- but refused to substantiate this claim to the Better Business Bureau of Metropolitan New York. Some clubs will send a local courier to pick up cash payment for your membership, if you stubbornly refuse to divulge any details about your credit card.

Wirsig of the Los Angeles BBB told my associate Virginia Wilson that he is so worried about the growth in "telecheating" that he's trying to persuade a legitimate travel company to back away from a telemarketing plan. He fears that it will give all the travel-club scams more believability in the public eye.

A bill now before the California legislature would require telemarketing boiler rooms to register with the state's Department of Justice before they go on line with consumers, which would give law enforcement officers a modest new tool to use against teletheft. But it's so easy for a company to go in and out of business that the law is usually one jump behind.

Besides California, complaints about travel-club telemarketers have been raised in Florida, Utah, Colorado, New York and Texas. But it's usually tough for a state law-enforcement agency to do anything for bilked customers from out of state.

If you do get taken, report the unauthorized charge on your credit-card bill immediately. Under the law, your credit-card company can't charge you for any more than $50 of the loss; it has to swallow the rest itself. If you're inattentive, however, and pay your bill without checking it, the loss may be all yours.