After ringing five times, the answering machine picks up and says "I am not in right now. Please leave your message and I'll call you back."

That message, repeated millions of times a day in homes, now is echoing in cars equipped with mobile cellular telephones.

Cellular phones use radio signals that are automatically switched between transmitting areas, or "cells," as callers move from one location to another.

In Washington -- the second city in the nation (after Chicago) to get cellular service -- industry sources estimate there are about 7,000 users of cellular phones. That number is expected to reach about 70,000 in the next three to five years.

Most cellular phones are in cars, but they also come in a portable variety that mounts in a briefcase, and in walkie-talkie models that are getting progressively smaller.

Some cars these days have cellular phones wired to the horn, or to a flashing light on top of the car. That way, customers near their cars but not inside can hear or see that they have a call -- and so can everybody else.

Advanced cellular phone systems, run in conjunction with paging systems, can notify owners about a call left on an answering machine. The telephone takes a message, automatically activating a pager in the owner's pocket.

The same person can even send data to his company via a small computer terminal hooked up to the car phone, or can plug into a computer data bank at the home office.

General Motors Corp. is building cellular phones into its more expensive models including the Buick Riviera, and custom installations in Mercedes and Porsches are instant status.

Soon cars with computerized capabilities may be able to diagnose their own problems and the mobile phone will automatically dial in the problem to the American Automobile Association or to a repair shop, says Brian Wood, a spokesman for Bell Atlantic Mobile Systems, a subsidiary of Bell Atlantic Corp. that offers cellular service in Washington, Baltimore, Philadelphia and other cities.

But woe to the car-phone thief. Oki Advanced Communications, a division of Oki Electric Overseas Corp., reportedly is developing a car phone that acts like a security guard.

"You park your car . . . lock the car, and if someone breaks into it, the phone is supposed to dial the police department," Wood says.

The phones can be purchased with redial, speed-dial and "spill-proof" capabilities. Hold buttons, electronic locks, adjustable volume and self-adjusting dial brightness for night driving are popular accessories, says Wood. Speaker phones that clip to sun visors are available for customers who want to talk without taking their hand off the wheel.

Any one of these bells and whistles can be yours -- for a price. Mobile phones without all the accouterments known to man start at $1,500 and the extras can propel the cost past $3,500.

Customers who subscribe to cellular phone service pay several hundred dollars for installation plus a high monthly fee. Call waiting, call forwarding, conference calling, three-way calling, speed dialing and other goodies can push the fee into the several-hundred-dollar range. Depending upon the service provider, the phones can be leased, rented or purchased.

Those prices keep mobile phones safely out of the range of most buyers. "It's really primarily still 98 percent total business product," says Albert Grimes, vice president of Cellular One, a joint venture between The Washington Post Co., Metromedia Inc., Graphic Scanning Corp., Metrocall, and Metropolitan Radio Systems, which offers service in the Baltimore-Washington area and other cities.

Salesmen use the product religiously, says Timothy Price, vice president of marketing for MCI Airsignal, a subsidiary of MCI Communications Corp., which provides mobile service to Minneapolis, Denver, and Los Angeles. "The salesman's nemesis is the time he spends driving to appointments. Once he has a mobile phone, a salesman has valuable marketing time" and can add two hours to his business day, Price says.

"As far as I'm concerned, it makes two Jeff Ellises," says mobile phone user Jeffrey Ellis, president of Bethesda's Ridgewell's Caterers Inc. "It's my assistant, my aide, my comfort. I'm accessible, and sometimes too accessible, but you need it for service and that's the business I'm in."

Ellis uses the phone in his car to negotiate business contracts. He also has a portable briefcase phone for making calls from catering sites. The mobile car phone has gotten him out of tight spots more than once, while the briefcase phone -- which goes with him on weekends to his boat on the Chesapeake Bay -- has at times been embarrassing.

Ellis recalls being stuck in traffic and having to meet a major deadline. "Because I was accessible, we made the deal and it was probably one of the most important deals of my life." On the embarrassing side, "When the phone rings on the street, you put your briefcase down and answer the phone. People think you are Secret Service. I just say 'Ronnie's doing fine.' "

"It used to be a toy, now it's a valuable tool," says Joel Rosenberg, an owner of the Cedar Post casual clothing stores located in Georgetown Park, White Flint Mall, Tysons Corner and Columbia. "One time I was on the Beltway and a car had just caught on fire. I just called 911 emergency right away and saved the driver's life. . . . If I hadn't seen it, it would have taken time for somebody to call the station."

The mobile phone appeals to everyone from lawyers and doctors to clergymen and real estate brokers, says Ann Director, spokewoman for Telocator Network of America, an association for the cellular and paging industry.

Last month, the House Administration Subcommittee on Office Systems added rented portable phones to the list of gadgets congressmen can order with their office allowances. So far, five congressmen have put in orders, says Ray Colley, deputy clerk for the House.

"We wanted a phone that would be of value to the members when they're not in an automobile," explained Jack Carpenter, the subcommittee's staff director. "Many of them are flying back and forth to their districts, and this way they can carry the phone with them."

On the Senate side, mobile phones have been added to the cars of the majority and minority leaders.

As competition intensifies, phone prices are expected to drop below $1,000, and manufacturers claim they could become a standard household appliance within three years. "You are talking about a cellular phone a daughter will take to night school for safety reasons, and a wife will keep in a second family car," says Cellular One's Grimes.

The Federal Communications Commission, which is granting two cellular licenses in each of 330 cities, envisions a nationwide compatible system. The FCC has granted 30 licenses so far and accepted applications for permits in 120 cities since the service became available commercially in 1983.

About 16 major cities actually have service, according to the Cellular Communications Radio Association in Washington. And many companies are striking "roaming" agreements, which let them provide service to their customers over systems in other cities where the companies have no license.

Besides Washington and Baltimore, service is available in New York, Chicago, Philadelphia, Los Angeles, Denver, Milwaukee, Detroit, Atlanta, Miami, Buffalo, Indianapolis, St. Louis, Minneapolis, and Dallas, says Airsignal's Price.

Four million customers are expected to have service by next year, according to a study conducted by Arthur Anderson & Co., a Chicago accounting firm The same study predicts as many as 10 million subscribers in the top 90 markets by 1990.

Within three to five years, Washington will have 10 times as many cellular phones as today, industry sources predict. "Washington is a high-growth market . . . unique to us because it is the center of government," says Christine Davies, a spokeswoman for Bell Atlantic Mobile Systems. "And Baltimore is, too, because it is now the top convention city in the U.S."

But some analysts doubt the potential for growth is that great, saying the market may become oversaturated.

Consultant Herschel Shosteck, of Herschel Shosteck Associates, estimates that about 125,000 cellular phones will have been sold through 1984, and about 226,000 through 1985. "The real market was and is in major cities and the market is becoming quickly saturated," says Shosteck. After service begins in the 30 top markets in mid-1985, sales will decline because smaller markets will have far fewer businesses, he said.

"We're working with very small numbers," says John S. Bain, an analyst with Shearson Lehman/American Express Co. in New York. Bain says market studies indicate that for every network built, it takes three to five years before the penetration rate equals one mobile phone per 50 people, or 5 to 6 percent of area businesses.

"This is not something you just plug into your cigarette lighter," Bain says. "Before you get to the main populace, you've got to see the cost come down quite a bit."

"People have been living without this for a long time," Shosteck says. "It's not like food, sex or God. If you're making a hundred thousand a year, have critical decisions to make about keeping a sailboat, horse, sports car, and how many times you go to Europe, a cellular phone competes with all of those."