If it's not one of Murphy's law, it should be -- "Life's most important phone calls come while you're out."

Who knows what universe-altering contact you missed when you were out walking the dog? Your Rich Relative . . . Mr. Right . . . The Woman of Your Dreams . . . Reagan's Transition Team.

Fear of missed calls is one reason sales of home telephone-answering machines are booming. Price reduction -- with some models selling for less than $100 -- is another.

"One third of our retail sales are now for home use, compared to almost nothing five years ago," says Peter Grant, vice president of Code-A-Phone, which claims to be the world's largest manufacturer of telephone answering machines.

"Our estimate -- which may be optimistic -- is that 5 percent of American homes are so equipped. By 1990, I'd bet 35 percent of homes will have them."

Many new purchasers are women, says Bob Letendre, owner of Bethesda's Telephone Equipment Center. "More women working," he says, "means less people at home to answer the phone.

"People everywhere are busy these days -- and hard to get hold of -- especially in Washington.Apartments used to have secretarial services, but not as many have them anymore.

"People say answering machines are very important to their social life. They're good for business, too. One person swears he made $10,000 as a result of a message left on his machine.

"And it can be a status thing . . . 'So and So has one, so I'll get one, too.' I hear more and more people saying, 'Gee, I hate those things, but . . . '"

Telephone answering machines range in price from about $60 up to about $1,500.

If you're considering one, check to make sure it is "registered" with the Federal Communication's Commission: It should say so on the package. If it's not, you must arrange with the telephone company for a "protective coupling arrangement" that can cost several dollars per month, plus installation.

The four basic types of machines:

1. Simple answering device -- Answers the phone with a brief message and records a short (30- to 40- seconds) incoming response -- under $150.

2. Extended message recorder -- Will record an incoming message for as long as the caller talks -- up to $300.

3. Remote control -- Includes a "tone generator" that you sound into any telephone to activate your machine and play back your messages -- $350 and up.

4. Remote annoncement changer -- Allows you to re-record and update your message while you're away from your phone -- $400 and up.