MIAMI — Toll telephone sex lines have been around for decades, but child advocates and law-enforcement officers say a different kind of chat line is gaining in popularity — those with local numbers.

And these free, unrestricted numbers are making it difficult for parents to monitor and easier for underage teenagers to fall prey to sexual predators.

Although the service is advertised for those 18 or older, critics say there's no one minding the store. On these lines, women call for free and men must pay with a credit card. But they generally stay on just long enough to give out, say, their cell number to stay in touch.

That's what happened with a 13-year-old Miami-Dade County girl, the daughter of a police officer, two years ago.

Bored and curious, she dialed into a telephone chat line, just for fun, ignoring the 18-and-older warning.

Lonnie Brady, a man more than twice her age, picked up on the other end.

The two made plans to meet in person.

After a few more conversations, the teen, who admitted she lied about her age, agreed to meet Brady, one summer night in 2005.

She lost her virginity in his car.

"Even though I lied about my age, he shouldn't have tried to talk to me,'' the teen said in court records, before Brady, of Florida City, was found guilty of lewd behavior in April. "I just don't want any other girls to do that, when they're not ready.''

NO RECORDS, NO OVERSIGHT

While law-enforcement agencies have noticed a spike in the number of underage teens using the service in the last three years, it is difficult to get a handle on how big of an increase it is, since Internet child predators are commanding the most police attention right now. Telephone chat lines rarely register on law enforcement's radar.

These cases are especially perplexing because they are hard to prove, and unlike the Internet, the conversations don't leave a transcript. Teens are often reluctant to come forth because, police say, they often think they're in a romantic relationship.

Shows like "Dateline: To Catch a Predator" focus on nabbing men who troll the Internet for underage prey. But the idea of meeting strangers on the telephone has not drawn the same kind of attention.

The Federal Communications Commission regulates telephone sex lines, requiring them to have telltale 900 or 976 numbers, which also charge by the minute.

But the local chat lines, springing up around the country, do not have such oversight, said Lance Cooper, a Georgia attorney who is suing a chat-line company for allowing underage teens to participate. His client is a 15-year-old girl who said she was raped by a man she met on the chat line.

"These party lines are like happy hours at the local bar,'' Cooper said. "But no one is checking for the female's identification.''

Also muddying the issue: The conversation on local chat lines can be as innocent as chitchatting about how someone spent the day.

The chat lines are "a poor person's Internet for kids who can't afford computers,'' said Parry Aftab, a New York lawyer who tracks Internet crimes.

"THIS IS HOW THEY CYBER-DATE.''

The lines appear to be most popular in rural and low-income urban areas, Aftab said. Already, she's discussing the lack of monitoring with the Ohio and New Jersey attorneys general.

Chat lines are advertised on late-night television, featuring attractive young women talking about how the numbers are the latest way to meet cool friends.

At least three Miami-Dade court cases have surfaced so far this year dealing with teens having sex with people they met on telephone chat lines.

In the case of the 13-year-old Miami girl, she confided to a relative, who told her mother.

Brady, 32, was convicted in April of two counts of lewd or lascivious battery on a child. He was sentenced to a year in jail.

Last year, another Miami teen, a 15-year-old freshman at Miami Northwestern High School, left a message over a gay chat line, saying, ''If you like what you hear, let your boy know what is up.''

Toriano Youngblood, 26, of Miami, answered. One day in May 2006, he drove up to the school in a gold Maxima with 22-inch rims and picked up the youth, who was on a school lunch break. They went to Youngblood's duplex and had sex, according to court records.

Afterward, the teen told a school social worker, who called police.

NO CITY IMMUNE

Similar problems have occurred around the nation:

— In Chicago, at least five murders and two rapes of teens occurred after they met their attackers on a telephone chat line, according to police.

— In the Boston area, at least 15 girls told law-enforcement authorities they were raped or assaulted after meeting men on chat lines.

Cooper, the Georgia lawyer, is suing to shut down chat-line operator Quest Personals because its owners do not screen for underage users.

Quest runs a telephone chat and dating service with more than 1 million members nationally in 225 locations. It's owned by a Canadian company, First Media Group Inc. The company could not be reached by phone calls and e-mails.

Over the last three years, Sgt. Thomas Sims, of the San Jose, Calif., police task force on Internet crimes against children, says he has seen about 16 cases.

"A lot of times, these children are outcasts or don't have social skills other kids have,'' Sims said. "They try to develop social skills on chat lines.''

PREVENTION TIPS

Here are a few ways for parents to prevent their children from talking on chat lines. They come from New York attorney Parry Aftab, executive director of WiredSafety, an agency dedicated to helping victims of cyber-abuse, especially children.

— Parents should pay attention to their kids' activities. If parents find that children are being more secretive than usual, spending more time than usual on the phone or having strange people calling them, they should investigate.

— Be more connected with your children. The more open the communication is, the less likely they will turn to strangers for love, affection and attention.

— Parents need to talk frankly about the consequences of risky behavior, whether it involves talking on the phone with strangers or meeting adults at the mall or at school.

If a stranger is trying to get close to a child, it often means the child is being groomed for sexual reasons, Aftab said.

Parents should teach children that relationships with adults they don't know will most likely end badly, Aftab said.