(MONEY Magazine) – CALLERS TO OUR NATIONAL RADIO SHOW AS WELL as securities regulators we've recently interviewed reveal the same thing: Americans are losing hundreds of millions of dollars in three growing investment scams. Before we go into the details, we want to remind you of one important money-management rule. Anytime an investment sounds too good to be true--for instance, a fabulous return over a short time frame with little or no financial risk to you--chances are you're being sold something bogus.

Lately, though, some con artists have refined their spiels so well that it's hard to tell if you're being pitched a scam or a genuine opportunity. Take it from us: Don't fall for these three sales pitches, typically made by telemarketers:

1. Make money selling your credit history. This scam can sound like an easy way to pull down extra bucks if you have a sparkling credit record. You apply for a loan or second mortgage, usually of $25,000 or more, and upon getting it, transfer the loan obligation to the investment firm or its client. The client agrees to pay off the loan. In exchange for feeding on your credit history, you get an instant $500 to $2,000 payoff.

Don't you dare go for this one. Even if the client signs a legal document saying he'll repay the loan, you're still the one on the hook with the lender. You may well get the $500 to $2,000 tip--quickly followed by the entire debt as the con artist hits the road with the cash.

2. Buy an interest in a 900-number information-provider partnership. In this scheme, you're told to put up a minimum of $5,000 to $10,000 in exchange for an interest in a 900-number partnership. Instead of owning outright a 900 number that charges callers as much as $5 a minute or $25 a call for information or advice, your money is pooled together with other investors, similar to a real estate or oil and gas limited partnership. Investors are often told they can double their money in a year. What you're not told, however, is that the people setting up the 900 service will charge enormous amounts to cover promotion, advertising, marketing, leasing costs and endorsement fees. Start-up costs can easily top $50,000. You probably also won't learn that roughly 80% of 900 numbers fail, so that you shouldn't buy any 900 service unless you're prepared to lose every penny you put in.

3. Invest in a Federal Communications Commission license to operate a regional paging service. First, the scammers touted deals selling you radio frequencies. Then, it was wireless cable. Next, cellular-phone licenses. In the latest communications swindle, a con artist offers to secure you a license covering a specific regional paging frequency; all you have to put up is a $2,000 to $12,000 application fee. The payoff: supposedly anywhere from $1,000 a month to several hundred thousand dollars. The pitch goes like this: "You can make a fortune leasing your FCC license to major paging companies because the FCC allows each company to obtain only one license in any market."

Wrong! There is usually no limit to the number of licenses a paging company can have in any 70-mile-radius market. What's more, the FCC doesn't preclude any company from having more than one license in the same market. Since paging companies can buy as many licenses as they want, they're certainly not interested in paying someone (namely, you) to lease a paging license in order to build their customer base. The bottom line: Federal Trade Commission attorney Gregg Shapiro says investors have probably lost as much as $200 million on these pager deals since 1994.

You now know to hang up on a telemarketer offering one of these schemes. But if someone you don't know calls with another investment opportunity, do some research. For instance, call your state's securities division to see if the agency is familiar with the investment and the firm. Also, visit the FTC's Website on the Internet (http://www.ftc.gov), which offers the agency's telemarketing rules and press releases about recent cases it filed, as well as warnings about scams. The more you know in advance, the safer you'll be.

The authors write the newsletter Straight Talk on Your Money ($75 a year; 800-777-2002), co-host a daily personal-finance call-in show on the WOR Radio Network and are regular contributors to CBS This Morning.