TUCSON, Ariz. -- Michele Yontef, a private investigator also known as "Ma Bell," usually spends her time electronically searching for missing children or suspects wanted by the law.

In 2000, she found a child missing for 10 years -- just two hours after stumped law-enforcement officials handed her the case. She needed just a few days last year to find a deranged Oregon mother on the run with her kidnapped children.

"I'm a pit bull," declares the 38-year-old divorced mother of two whose Web site is ma-bell.com. "When I bite in, I don't let go."

These days, Ms. Yontef is gritting her teeth over her own legal problems. She is fighting a descendant of AT&T Corp. over her use of the "Ma Bell" name, the popular nickname of the giant AT&T monopoly that was broken up by the federal government in 1984.

Qwest Communications International Inc., one of several regional phone companies that inherited rights to the Bell name and logo, sued Ms. Yontef in federal court in Arizona last year, charging her with trademark infringement. A trial date is expected to be set this month.

The dispute is heating up just as AT&T is about to disappear upon completion of its acquisition by one of its progeny, SBC Communications Inc. Only one of the big four regional phone companies, BellSouth Corp., still uses Bell in its name. Qwest's Web site makes only brief mention of the company's more than 100-year history as part of the Bell system, named for the inventor of the telephone, Alexander Graham Bell.

These days, Qwest and its siblings don't even like to be called "Baby Bells." They say they have evolved into modern telecommunications companies that offer fiber-optic-based Internet and television services, not just phone connections. "We don't want to be thought of as a local phone company any more," says Janis Manning, assistant general counsel in charge of trademark issues for Verizon Communications Inc.

But that doesn't mean the companies are giving up their inheritance. "The tradition of the Bell brand and the legacy of service are very important to us, and we consider them a valuable asset," says Christine Searls, a senior attorney at Qwest.

She and other lawyers from the regional phone companies hold a monthly hour-long conference call to plot strategy, brief each other on cases and share information about possible new infringers of the trademark.

A Web site dedicated to the trademark and jointly run by the phone companies warns that they have defeated "literally thousands" of infringers, and that defendants' median cost for trademark cases that go to trial is $128,000, which "means half the cases cost more!"

The Ma Bell name has popped up in some unlikely places over the years. A New Mexico company sold a board game under the name in the 1980s. Around the same time, a pastry shop called Ma Bell's Buns opened in New York City. And a Colorado woman used the name for her ceramic figurine business in the late 1970s.

A BellSouth spokeswoman doubted that these companies had been sued for infringement, though she didn't know for sure. She says the companies target businesses that they think are telecom-related.

Alleged infringers typically give in quickly when contacted, according to the phone companies' lawyers.

In January, a Melville, N.Y., entrepreneur received a threatening letter from BellSouth, a day after his new venture, dubbed Bellster, was mentioned in a Wall Street Journal story. The venture enables users to make free phone calls through a computer. Founder Jeff Pulver dropped the Bellster name within a week after receiving the letter, he says.

Contributors to his blog encouraged him to fight the demand. "No one wanted me to give in," says Mr. Pulver. "Nor did anyone offer me $300,000 to fight the case."

Qwest contacted Ms. Yontef in early 2003 to demand that she change the name of her Web site, offering to pay her $30, the cost of registering the domain name. In its suit, Qwest claimed that Ms. Yontef was treading on its trademark because she "provides consulting and training on telecommunications matters."

Ms. Yontef says she used to teach phone sleuthing seminars but denies that she ever competed with the Denver-based company. She rejected the settlement offer.

"There is a principle at play here that I want my kids to understand," she says. "If a bully wants your lunch money, you don't hand it over -- you fight." She wonders who will be the next target. "Are they going after Taco Bell?" the fast-food chain, she says.

Ms. Yontef worked as a nurse, a telemarketer and as an answering-service operator before she developed her skills at collecting hard-to-get information. She says that during her divorce proceedings in 1998, she amassed information about her then-husband that won her custody of her children.

After going into the private investigation business in 2000, a client began calling her "Ma Bell." She says she liked the name because her father used to sing to her the Beatles song "Michelle," which includes the phrase, "Michelle, ma belle."

Ms. Yontef built a name for herself in part by giving phone-tracking seminars to other licensed private investigators around the country, including tips on everything from tracing cellphone calls to finding unlisted numbers.

She stopped the seminars two years ago to spend more time with her small children, she says.

To supplement her income, she testifies as an expert witness in court cases that involve phone records and other electronic evidence. She also serves legal papers on defendants, driving her purple 1992 T-top Camaro with the hood removed to cool the engine in the Arizona desert heat.

On a recent morning in her home office, several cats prowled around the three computers and numerous telephones cluttering up her desk. When a sensitive phone call came in, she flipped on a small light outside her door, signaling her kids to be quiet.

"Oh, hello sheriff," said Ms. Yontef. The Oceanside, Calif., officer was searching for a witness to a recent hit-and-run accident.

Ms. Yontef rapidly punched the witness's name into several Web sites. Within a couple of minutes, she found the individual's home address and telephone number and passed them along.

Ms. Yontef says much of her time now is spent studying law on the Internet so she can defend herself against Qwest. She says she can't afford an attorney.

In June, Qwest raised its settlement offer to $5,000. Ms. Yontef declined.