When Exotron Corp., a Princeton, N.J., manufacturer of ultrasonic and electronic devices for treating neuromuscular disorders, made the decision to move from research only into sales, it needed a way to distinguish itself from its competitors. Most suppliers of this specialty equipment are "Monday-through-Friday, 9-to-5 type operations," says Joseph Grossman, the company's director of operations. Being on call 24 hours a day seemed the best way for Exotron to gain a foothold in the market.

Conventional "beep" pagers were issued to the company's 19 sales and service personnel, who could then be signaled by Exotron's answering service any time of the day or night. However, for the hospitals, doctors, physical therapists, and sports teams that account for Exotron's $1 million in annual sales, service means more than machine maintenance and repair. Many of these customers have urgent, detailed questions about treatment procedures for Peter Amiet, the company's president and a specialist in therapeutic medicine.

Amiet, who spends much of his time lecturing at hospitals and out-of-state medical schools, needed something more sophisticated than a beeper. The answer was "voice retrieval," a relatively new development in the rapidly advancing technology of mobile communications.

Depending on his location, Amiet carries either an ultrahigh frequency (UHF) pager that operates over an area of approximately 200 square miles but emits only tone signals, or a very-high frequency (VHF) unit, more limited in its range but equipped with a liquid crystal display (LCD) screen to show either a telephone number or a single-digit code. When either the code appears on the VHF pager or he hears a special tone from the UHF unit, Amiet knows a voice message has been received at an electronic message center operated by his paging service, Radiofone Corp., of Englewood Cliffs, N.J.

This "voice mailbox" can be reached only by a special telephone number he has distributed to about 100 high-priority customers. Amiet receives the signal within 10 seconds of a call being placed to that number, alerting him to contact the center.

Pagers capable of delivering messages in the caller's voice have been available for some time. But the technology has new flexibility. Unlike voice-transmission pagers, which digitally encode a message and then play it out once, voice-retrieval systems store and replay the message as often as the user wants to hear it. The feature is particularly appreciated by those who have lost vital parts of sentences while straining to listen over the sounds of traffic or through the interference caused by high buildings.

The cost of the voice-retrieval option varies slightly according to the supplier. Radiofone charges Exotron $10 a month, in addition to the $23 and $33 fees that the company pays for its tone-signal and numerical-display pagers, respectively. Although providing this service for three executives adds $30 a month to the company's paging bill, bringing the total to about $450, Grossman considers it a bargain. "The cost," he says, "is still only a fraction of what we pay for conventional answering services."

Voice retrieval also adds an element of two-way communication to paging. Unlike mobile telephones, another evolving branch of telecommunications, paging wasn't designed as a medium for conversation. A voice mailbox offers the next best thing, letting users not only pick up calls, but also leave messages, which they can alter as frequently as they want from any location.

Currently, the geographic mobility of pager users is limited by the short signal range of paging companies, which are tied into the lines of local telephone companies. Incoming messages are electronically coded into signals that can be broadcast via transmitters, which have an average range of only about 25 miles. That means wide-area signaling, such as Amiet enjoys, is available only when a paging company either owns a large number of transmitting facilities -- Radiofone has 109 scattered over several states -- or has been able to work out cooperative arrangements with other companies that hold broadcasting licenses and operate transmitters in adjacent territories.

Barriers to long-distance paging are falling rapidly, however. Radio Common Carriers (RCCs) -- the hundreds of federally licensed companies that provide paging, car telephone, and portable-telephone services -- have traditionally tended to be small, entrepreneurial operations. But a surge of subscriber demand for paging services and the anticipation of a revolution in the mobile-telephone market is fast attracting bigger players to the industry.

"We are guessing that, within two years, we'll see the whole common-carrier industry turn over," says Jerry Taylor, president of MCI Airsignal Inc., a corporation formed after MCI Communications Corp. acquired WUI Inc. Taylor's company is a good example of the changes he predicts. The purchase of WUI and the formation of its Airsignal subsidiary gave MCI an immediate base of RCC licenses in 52 cities. MCI Airsignal is filing for many more common carrier license permits, as are other large corporations, such as Washington Post Co. and Cox Communications Inc. One of the most aggressive has been Metromedia Inc., which has purchased five RCCs over the past year, including Radiofone. Several Federal Communications Commission decisions and one major technological advance -- cellular radio -- are behind the increased interest in RCCs.

Paging devices and mobile telephones have existed for decades. But the capacity to provide service to users has been limited by the finite number of radio frequencies, or channels, that exist for broadcasting either one-way (paging) or two-way (mobile and portable telephone) signals. Last spring, however, the FCC drastically altered this situation. Forty new channels for paging applications were opened up -- three of them specifically set aside for regional or nationwide paging through either a network of cooperating RCCs or, possibly, satellite transmission by a single company. At the same time, the commission allocated 666 previously unused UHF channels for licensing to operators of cellular-radio services.

The technology for cellular radio is radically different from that used previously with mobile phones. Instead of relying on powerful transmitters to broadcast signals over an entire metropolitan area, cellular operates on a grid system. A region is divided into sections, or cells. As the user travels from section to section, a signal is passed along from one low-power transmitter and one short-reach channel to another. Computerized switching devices eliminate traffic jams on frequencies in much the way synchronized traffic lights ease auto congestion on city streets.

The technology, therefore, allows the use of channels that previously wouldn't have been suitable for mobile telephone communication because the range of reception was limited. The grid system also ensures maximum efficiency on each channel. For these reasons, cellular radio, at least theoretically, provides enough channels to meet pent-up demand.

As the number of subscribers and equipment manufacturers rises, rates should fall substantially. Wayne Schelle, president of American Radio-Telephone Service Inc., a Baltimore-based company that is test-marketing cellular radios to 200 customers, says that trial prices are $45 a month for service, $100 a month for equipment rental, and 40 cents a minute for calls. Eventually, he expects to charge as little as $10 a month for service, $55 to $60 a month for rental, and 25 cents a minute for calls.

William Peterson, president and owner of Professional Sounds Inc., in Fairfax, Va., a company that designs and furnishes equipment and professional audio services, sees himself as an example of the prime prospect for the emerging telecommunications technology -- "the smaller businessman who has to be out hustling and talking to people but still has to be the kingpin of the operation." His user credentials are already impressive: He has one conventional car-installed mobile telephone, tone-voice pagers for each of his company's six employees, one numeric-display pager, and has participated in a test program of a cellular-radio system using American Radio-Telephone's interconnect service.

Peterson considers the $200 a month he pays for his portable phone a good investment. "If I can make just one call and get a piece of business, or if I can service one important client who wants to speak with me now, that's worth $200 a month to me."

The ability to respond quikly to customers has been the strongest motivation for subscribing to mobile-communications services, whether telephones or paging devices. But the new technologies are creating new applications.

Pagers, costing as little as $35 to $45 a month, already spell out messages in letters and numbers (alphanumeric displays), using an LCD screen. In a few more months, the devices will print out copy on paper the size of ticker tape. And perhaps as early as the end of this year, units will have keyboards, allowing them to function as terminals through which users can tap into databases. Eventually, paging companies will offer equipment that enables subscribers to hear, in computer-synthesized speech, messages that their callers have typed using the buttons of their telephone.

"Paging is in an embryonic stage," says MCI's Taylor. Eventually, he envisions pagers "delivering any kind of information that can be broadcast and delivering it anywhere in this country or abroad."

David Post, chairman and chief executive officer of Page America Communications Inc., a New York City-based paging, message, and information network and holder of more than 20 RCC licenses, has forged a merger agreement with Page World Communications Inc., a company involved in joint ventures with RCCs in Western Europe and Israel. Post envisions a world in which data from many sources will be delivered into people's pockets. "Imagine," he says, "that a sales manager finds out from the head of operations at the factory that there has been a production overrun, and the company has to move 12,000 boxes of socks. He goes over to the telex machine and punches in ALL SALES PERSONNEL. . . . "

It didn't happen quite that way for Harry Baltimore, but the scenario was dramatic enough to convince him that he didn't want to do without the PageGram pager he had rented from Page America for a three-month trial. Baltimore, director of operations at CYE Industries Inc., a 30-employee, garment-tag manufacturer in New York, was having lunch in a restaurant when his pager beeped and the display printed out his office telephone number. When he called the office he learned that a potential customer had phoned with a rush request for a job estimate -- the customer needed the figures that afternoon. Baltimore jotted down the details, did some fast calculations and called the client back -- all without moving from the restaurant. CYE got the job, and PageGram gained a convert.

Baltimore had never been too happy with the idea of wearing a beeper, he says. It was the numeric display that finally convinced him to try a pager for $28 a month. "I liked the fact that by printing out telephone numbers the unit gave me immediate access to my messages. With all the paging systems I had seen before, you had to keep checking your office to see who was calling."

Baltimore isn't alone in his initial resistance. Many executives have been conditioned to think of pagers as nothing more than summoning devices -- useful only to surgeons on the 18th hole and service personnel. Motorola Inc., a major equipment manufacturer, is attacking these preconceptions on two fronts. While adding memory and alphanumerics to some of its units, the company is also putting its money on snob appeal in the form of Sensar, a tone pager that is about the size and shape of a ball-point pen. "It looks just like my Mont Blanc," says American Radio-Telephone president Wayne Schelle, who plans to offer the new model to his subscribers for about $300.

Post remains convinced that it will be informational capabilities, rather than cosmetics, that will break down executive resistance. "There's a big difference between being bothered and being fed information," he says. "We'll soon be seeing field-service people using beepers and the salespeople or executives using more advanced models."

That, in fact, is already happening at Exotron, where the unit an employee carries provides a quick reading on that person's level of responsibility. "As an individual reaches a certain standing within the company, we will move him up from a conventional beeper to the telemessage service," says Joseph Grossman, providing what may be a glimpse of the latest trend in corporate status symbols.