A Capitol Hill lobbyist was discussing unemployment insurance with a congressman when a small voice squawked, "Call your mother."

The lobbyist's paging device had broadcast a recorded message left on her office telephone.

"Sometimes it's embarrassing," said Hilary Rosen, a lobbyist who witnessed the incident and also carries a pager. "But we depend on pagers so much. You just don't operate without them."

An estimated 3 million people in the country now carry electronic message machines in their pockets.

More than 100,000 beepers are in use in the Washington-Baltimore-Annapolis area, the nation's fourth-largest market, said Stan Krejci, vice president and general manager of Metrocall Inc., an Alexandria telecommunications company. The number of pagers in the area has doubled in the last five years, and the pace of sales is quickening, he said.

Using radio frequencies to play recorded messages, display written information or simply beep, the increasingly sophisticated pagers make up one of the fastest-growing sectors of the booming personal telecommunications industry.

By 1990, about 10 million personal mobile message machines, or pagers, will be in use nationwide, constituting a $3.6 billion-a-year business, said Clifford A. Bean, a telecommunications industry analyst with Arthur D. Little Inc. of Cambridge, Mass.

Electronic pagers have been available since the late 1940s, and traditionally were used as emergency alert devices carried by doctors, ambulance drivers and plumbers. Today, white-collar managers and professionals, particularly upper-level executives, are the biggest source of industry growth, Bean said.

Sellers and users of the mini-message systems describe them as an extension of the business phone, an electronic lifeline allowing the user to venture out of the office while providing a constant connection to an information nerve center.

"I think of it as an umbilical cord to the Capitol," said Rep. Claudine Schneider (R-R.I.), who carries a pager to keep track of House votes while she attends meetings or runs errands. "It helps me to use every minute of the day."

"The person without a pager, seven to 10 years from now, will be like the person without a business phone today," Krejci said.

One reason for the growth of the industry, observers agree, is the improvement of the technology. The traditional beeper emits a tone, or "beep," notifying the user to call the office or home because a message is waiting.

Today's mobile message machines do more then sound off at embarrassing moments. Some display numbers -- a telephone to call, a time to meet or a predetermined code. Others have screens like those on calculators and display short messages. All have memories to store the messages, and some can be hooked up to printers.

Some pagers broadcast a message recorded on a telephone answering machine, and some forward messages from car phones.

Most pagers can be used in a "silent mode" -- with the beeper shut off -- alerting the user by vibrating. Some look like small calculators and are worn on the belt, while others look like large designer pens and can be worn in a shirt pocket.

One major improvement in the equipment is the price. The simplest tone-only pagers now sell for slightly less than $100, compared with about $300 two years ago.

Metrocall, which sells its Hippobeepamus tone-only pager in retail outlets such as Woodward & Lothrop stores, prices the device at $98 or leases it for $49.95 a year. Service then costs $9 a month for unlimited use, or $4.95 a month plus 20 cents per call. The more advanced models cost considerably more.

MCI Airsignal, a subsidiary of MCI Communications Corp. that sells its beepers through Radio Shack stores in some cities, says its equipment costs between $90 and $200 while monthly service fees start at $6 a month.

Another big reason for the growth of the market is deregulation of FM radio frequencies by the Federal Communications Commission. In January 1982 the FCC allocated more frequencies for use as paging channels.

The FCC move made room for new players, including large national telecommunications corporations, in a business that had been the preserve of small, family-owned businesses across the country.

The increased competition has led to more aggressive and innovative marketing efforts, aimed at increasing public awareness of the uses and benefits of paging systems, Bean said.

"People have thought of pagers as a leash to the boss," said Tim Price, vice president of sales and marketing for MCI. "We say it liberates you from the tyranny of the office phone. . . . It's a great opportunity maker for anyone who values their time and wants to increase their productivity."

Ron Prescott, a Sacramento lobbyist for the Los Angeles Unified School District, said it saves him the time and energy of calling the office every hour to check for messages.

"I'm freed," Prescott said. "Time is of the essence in a political environment. Things happen quickly and I have to respond. . . . Now I know I can be reached and don't have to continually check in."

MCI is focusing its sales efforts on the business user, "essentially adult, white-collar males," Price said, adding that the marketing approach corrals businesswomen as well.

Any business professional, particularly those in service industries with clients to serve and meetings to attend, can benefit from pagers, he said.

"Washington is absolutely the perfect market for pagers" because many professionals have to leave the office to meet with government representatives, Bean said, pointing to the mobility of politicians, congressional aides, lawyers, lobbyists and reporters constantly on the move from offices to hearings to meetings.

Congressional staffers agree that Capitol Hill is filled with beepers and beeper addicts, sometimes causing a cacaphony of tones and recordings in cafeterias and hearings.

Certain members of Congress are known to stare down indignantly lobbyists if their beepers go off during hearings. Other members won't leave the office without one.

Schneider said she sleeps fully clothed with a beeper on her pillow when important House votes get postponed late into the night.

"We beep her all the time," said Steven Provost, Schneider's press secretary. "She's always moving all over. . . . I don't know how we would survive without it."

Metrocall is trying to expand sales to the consumer market with its Hippobeepamus campaign, launched a year ago, and says it is "very pleased" with the results.

"The market needs educating," said Krejci of Metrocall. "They need more information about what a pager can do."

Other companies and analysts say market studies show the pager still is primarily a business tool.

Bean, the telecommunications analyst, said Hippobeepamus "was a dismal failure" as a consumer product, and is being used by small businesses as a low-cost alternative to other beepers on the market.

"It's still fundamentally a business device," Bean said. "And it will continue to be paid for out of the business budget."

MCI believes pagers will go the route of personal computers, with users becoming familiar with the device in a business context and then thinking of personal applications.

"We want to penetrate the consumer market via the business market," Price said, predicting that pagers will become consumer products by 1990.

A single pager can transmit calls not only within one city but also within areas as large as the state of California.

However, by 1986, nationwide paging networks are expected to be operating, says Telocator, a national trade association that represents firms providing paging and mobile telephone services. Nationwide paging will mean that phone calls to a beeper used in one city will be forwarded automatically to every other city on the network. Beeper users will be able to return calls without the caller even knowing where they are.