WILLIAM Becker, 19, passes his summers in the basement of his family's home in the Washington suburbs, at a worktable between the clothes dryer and the water meter, making telephones.

This summer it's 70 Plexiglas phone shells for a local retail electronics firm. It'll take him about two weeks' worth of heating, molding, measuring, sawing, sanding, buffing and gluing.

The result will be long, clean-lined shells with the receiver cradle to the left of the body instead of on top, a departure from the lop-earned dowdiness of the standard desk phone. Each (patent pending) shell will retail for $40.80, with a see-through model on the high end of the scale. Prefitted with a touch-tone mechanism, a Becker phone goes for $90-140.

"I started doing this maybe five years ago," Becker reflects. "I made a Plexiglas cassette rack. And when I found out how easy it was to work with plastics, how much could be done with them, I started experimenting."

The experiments, ranged around an upstairs room, are a desk intercom, a digital clock, the cassette rack and an infinity box, backed by a mirror, lit with tiny bulbs around the inside, giving the effect of the Jersey Turnpike curving into an eternity of sodium lamps on a rainy night.

"I decided to make phones because so many of the ones on the market seemed ugly to me. So I began to design, fabricate and market my own. There was a lot of trial and error involved at first. I started with poor equipment."

Today his studio includes a strip heater (a long electrified box which heats very narrow areas of flat Plexiglas sheets), sets of jigs (plastic squares mounted upright at precise angles, on which the heated Plexiglas is bent to the right shape), table and jig saws and a sanding and buffing machine.

Becker speaks in the assured tone of someone who's acquired expertise in his craft a bit faster than most of us could get a grip on the basics.His gaze is fixed on a tangent to the circle of conversation.

"I made some plastic display racks for a few stores, and at first I made only seven or eight phones a year. I started selling them through electronics stores. Now I work about four months a year, and during the other eight months, my business a pretty much dead."

Only because, during the other eight months, he's at Syracuse University studying for a degree in industrial design.

"I stay with one design at a time, since I have very little space to work in. I've done custom work for relatives, and some stores. But that's a lot of work - even these are," he gestures broadly at the worktable.

"I like to design, and most of the time I'll develop a new model spontaneously. Or sometimes a store says, "This one's not selling too well, you've got to redesign it.'"

But one design influence he disregards is that of the phone company.

"No, I don't pay attention to them. A lot of their phones aren't well-designed. They're kind of cheap-looking, molded-looking. They look mass-produced.

"I don't know if designer phones will still be big when I get out of school. Up to now, people weren't able to hook them up without a hassle. Now it's a lot easier, and designer phones are part of the fad of expressing the individual's personality. Maybe in a few years the designer phone will become commonplace and there won't be so much of a fascination with it."

Plexiglas phones are durable with normal use, Becker says - but normal use does not include accidently dropping one on the floor, which seems to happen to most phones in kitchens and rec rooms at least once a year.

"No designer phone is as durable as the ones you get from the phone company," Becker says. "Any designer phone - including Plexiglas - if you drop it, there's bound to be some kind of damage.

Plastic designer phones are best cared for, aside from keeping them away from ledges, by washing them with warm soapy water or the plastic cleaners sold by electronics firms and department stores, according to Becker. Don't use abrasives or scouring powders, he warns.

"How To Buy, Install and Maintain Your Own Telephone Equipment," by Joseph La Carrubba and Louis Zimmer (Binghamton, NY: Almar Press, 1978), takes advantage of recent Federal Communications Commission rulings that permit consumers to buy some phone equipment instead of renting it from the phone company.

La Carrubba and Zimmer are independent installers, and the main point of their 50-page booklet is that a phone is a simpler machine than you might think. What's complicated is the system the phone connects into, and that system (as La Carrubba and Zimmer point out) is protected by the FCC from encroachment by bad equipment.

Anything you hook into the phone network should be registered with the FCC. If the equipment isn't FCC-registered by the manufacturer, you need FCC-registered circuitry to protect the phone system. You can connect with the phone lines only by plugging into sockets installed by the telephone company.

And that's just the half of it. But despite all these caveats, La Carrubba's and Zimmer's book is the closest thing yet to a Motor's Manual of telephones, and easier to read, too.