FORT LAUDERDALE, Fla. -- Every week, millions of consumers buy prepaid calling cards, primarily to make international calls to family and friends. But many low-cost calling cards don't deliver what they promise, law enforcement officials say.

Prompted by more than 200 consumer complaints filed about calling cards last year, Florida Attorney General Bill McCollum launched an investigation into the marketing and business practices of 10 companies. Many consumers, including a large proportion of Hispanic immigrants, alleged the companies did not disclose hidden fees and misrepresented the number of calling minutes they received.

And last month, a U.S. district judge granted a Federal Trade Commission request for a restraining order against CTA, a major national distributor of prepaid calling cards. The FTC tested the cards and found consumers typically paid for more minutes than the cards provided.

Consumers have been "cheated" by these companies, said Gus West, president of the Hispanic Institute, a nonprofit organization. Prepaid calling phone cards are used to buy minutes in advance to make local or long-distance calls. Low-income people often use them because they offer a less expensive way to make international calls than a monthly long-distance plan. Consumers can pay from $2 to $20 per card, depending on the amount of time and the rate per minute charged.

Regulators and consumer advocates said competition in this $4 billion industry had led to deceptive practices. Some companies misrepresent the costs they advertise, often by charging hidden fees that reduce the number of minutes on prepaid cards.

Only about 60 percent of prepaid calling cards provided the minutes advertised, according to a study by The Hispanic Institute last year.

Prepaid calling card companies advertise low rates with no connection fees, often posting notices in small retail outlets such as gas stations, grocery stores, bodegas and newsstands that sell cards.

Rep. Eliot Engel, D-N.Y., has proposed legislation to regulate the industry after he used a calling card and got fewer minutes than he purchased. The Calling Card Consumer Protection Act would make it illegal for a provider or distributor to impose any charge or fee that is not fully disclosed. The bill also would explicitly prohibit calling-card companies from providing fewer minutes than they disclose.

"It is morally wrong to cheat anybody," Mr. Engel said. "[Companies] can make money by telling the truth."

Industry leaders such as IDT Telecom Inc., of Newark, N.J., and its distributor, Union Telecard Alliance, based in New York, sued six rivals in 2007 to curtail deceptive practices.

Consumer complaints also have prompted state and federal investigations.