Why IBM is plugging in on switchboards.

Sometimes it all seems so simple. The battle for the office automation market boils down to two things: your phone and your computer. Any company that can control both of these desktop fixtures will come out a winner, says Greg Carlsted, an analyst at Dataquest, a Cupertino, Calif., high-tech research firm.

Right now, the company closest to the lead is American Telephone & Telegraph. But last week, International Business Machines stepped closer. It announced plans to acquire the Rolm Corporation, which makes switchboards, called private branch exchanges. Pending federal approval, IBM will pay $1.26 billion for the company - the largest acquisition in IBM history.

Rolm, an entrepreneurial company based in Santa Clara, Calif., is a relatively small, but technologically advanced, player in the private branch exchange (PBX) market. At the moment, the market for switchboards as a whole is growing only moderately - just about every company already has one. The growth potential, though, is in companies switching from older AT&T systems to new digital systems, such as those made by Rolm.

Almost every major computer company has some kind of relationship with a PBX manufacturer. But only a few have gone so far as to actually invest in one, and none have been so daring as IBM to buy one out.

What's so appealing about switchboard manufacturers? Are others attractive enough to lure IBM-like investment?

Kevin Sara, an analyst at Northern Business Information Inc., a New York market research firm, explains it this way:

''The immediate concern is the proliferation of personal computers in corporate America. They are really limited in their use alone, but once they start linking together, they become very powerful information tools. The easiest way to do this is through the telephone.'' One computer tapping a data base in some far-off city, electronic mail zipping from one terminal to another - all this happens through a phone connection.

But of course there is also a strategic reason for computer interest in the PBX business.

''Here are two opposing groups battling for control of the desk,'' says Mr. Carlsted of Dataquest. ''The key is to line up with a partner to stand a better chance of infiltrating that department and getting your product out front.'' The reason for the alignment, he adds, ''is that manufacturers recognize that neither one of them in and of themselves has the resources'' to develop the telecommunications/computer market.

Rather than make a commitment, it appears that most computer companies are perfectly happy playing the field in the PBX dating scene. All kinds of PBX brands are installed around the country. The most common are by AT&T, International Telephone & Telegraph, Northern Telecom in Canada, and GTE Corporation. But they are not the only ones by any means.

''If you make a financial transaction (i.e., acquire part or all of a PBX company), it forces you to choose one,'' says Brad Stroup, spokesman for Data General, a Westborough, Mass., computermaker. Data General has agreements with several PBX manufacturers, including giant Northern Telecom and upstart Ztel in Andover, Mass.

''There are a variety of products in the PBX area, and they are not yet standardized,'' Mr. Stroup explains. The PBX market ''is in a state of considerable flux, like 18 horses going in different directions. To get on one horse doesn't make much sense to us now,'' he says, adding that ''eventually that won't be true.'' Computermakers like Digital, Hewlett-Packard, and Burroughs have similar philosophies.

But not Wang. Since the spring it has had an option to own as much as 30 percent of InteCom Inc., an Allen, Texas, PBX-maker. It already owns 20 percent, and the two have just come out with a joint product - a computer work station that combines a phone with a terminal, the latest trend in office equipment.

Although Wang still maintains agreements with seven other PBX manufacturers, it went the equity route with InteCom ''to focus our efforts and give tangible indication of our direction to the marketplace,'' says Bud Benton, manager of Wang's PBX vendor program. The move also stopped a costly drain on Wang's resources, which were being spent to develop a PBX system in house. Yet ''if we were to buy up InteCom,'' Mr. Benton says, ''I think it would destroy the entrepreneurial atmosphere of a company like that.''

Some analysts worry this will be the case with Rolm, which has developed a laid-back, casual corporate culture - quite different from the organizational strait jacket wrapped around IBM. And analysts say they aren't quite sure that IBM's move to acquire all of Rolm (it already has a 23 percent stake in the company) will necessarily cause a wave of similar acquisitions throughout the industry. One observer, who asked not to be named, felt that IBM was forced to make this acquisition because Rolm had products on the drawing board that would directly compete with IBM.

But no analyst is ruling out the possibility that other companies will seek direct investment in PBX-makers.

''I wouldn't be a bit surprised if there are not other acquisitions and mergers,'' says Alan Fross, vice-president of Eastern Management Group, another research and consulting firm, in Parsippany, N.J. He points out that PBX manufacturers ''are pushing just as hard'' as computer companies for some relationship. The smaller, start-up companies realize they must spend huge amounts of capital on research and development, he says, and what better place to get it than from a computer company.

For example, Mitel Corporation, in Kanata, Ontario, has publicly stated that its door is open to any interested suitors. Its stock is depressed now, and analysts say the company is ripe for takeover.