When writer Hollie Davies is out, God answers her telephone. Darth Vader takes messages for psychologist Lee Solow; Sparky the Dog barks greetings for a Virginia veterinarian, and Adrian I -- a futuristic-sounding female -- answers for marketing specialist Evelyn Levenson-Bell.

These people are among the increasing number of busy Americans who have discovered the benefits -- and annoyances -- of equipping their telephones with automatic answering machines.

"The good part is knowing you won't miss an important call," says 27-year-old Anne O'Leary, a Foreign Service information officer who bought an answering machine last spring. "But the frustrating part is getting callers who'd rather hang up than talk to a machine.

"It drove me crazy to come home, turn on the tape and get dial tone after dial tone with an occasional Bronx cheer. So I started putting creative messages together to entreat people to leave their name and number.

"I change them every few weeks to relieve the tedium of hearing the same recording over and over. It relaxes people, especially those who are real machine-o-phobes, and I get fewer hang-ups."

Creating novel phone-answering messages has become a nationwide pastime, claims San Francisco radio personality Carter B. Smith, who plays a different home-made message on the air each afternoon.

"They run from the matter-of-fact to the expertly-produced -- complete with musical arrangements and background vocals," says Smith. "Some people have their whole family come on and sing, some do impersonations of celebrities like Rula Lenska or Mr. Rogers, and some are totally obscene."

Listeners call in their favorites to Smith, who has aired messages from all over the country. "Folks in Washington, D.C., have come up with some dandy takeoffs on Nixon," he says. "They record things like 'You have 18 seconds to leave your message or Rose Mary will erase it . . . But I shall return . . . your call.'

"California people tend to leave more mellow messages . . . lots of jazz piano and surfing sounds. My favorite was one from a bondage massage parlor outside the St. Louis airport. But, of course, I couldn't air it."

Despite this wave of creativity, some callers still hang up.

"I'm one of those people who often hangs-up -- although I'm getting better at leaving a message," admits anthropologist Elliot Liebow, chief of the NIMH center for work and mental health. "I think a lot of subtle things go on when you call a person and a machine answers.

"People feel a combination of intimidation and resentment. It's a kind of put-down to get a recording, and that sets up a kind of tension. It puts you in a subordinate position, petitioning for a hearing.

"It's not intentional, perhaps, but it gives the feeling that the person you called is very busy and you're just another person making demands on their time. And there's an implicit high status associated with it -- particularly when the message is stuffy. I think i'd rather get no answer."

"Many people just aren't comfortable talking to a machine," says New York psychologist Elliot Seligman, who often asks patients to use tape recorders at home as part of therapy.

"They think they sound silly on tape and they don't like the feeling that someone can play their voice back over and over and laugh at them."

"A machine doesn't give any feedback," adds Virginia psychologist Randall Chambers, who specializes in the psychology of technology. "When you converse with a person they'll make sounds or laugh to encourage you to talk. But a machine just sits there."

"It all comes down to performance anxiety," contends Lee Solow, a California psychologist and part-time actor. "People call expecting to talk to someone and suddenly they're thrust on stage, with just seconds to spit out their message.

"There's a sense of urgency and the need to put together the vital information in a matter of seconds. They get mike fright."

But once you break the ice and leave your first message with an answering machine, says Solow, "It gets easier and easier. I take special pride in coming up with humorous greetings that encourage people to leave messages.

"I change them every couple of weeks and try to be topical. I had a Japanese voice during Shogun week and I've had Star Wars and a fake presidential debate. I've had suicidal patients call up and say my tape cheered them out of their depressed mood.

"In the five years I've had the machine I've only had one person call and say they thought my message was unprofessional and didn't want to work with me. I'd just as soon not work with someone who has no sense of humor."

Preparing tapes "is a great outlet for my acting interests," says Solow, who has recorded message tapes for a California company that markets them nationwide.

Called "We've Got Your Number," the firm offers four cassettes that each cost about $10 and contain 12 messages. Among their best sellers are impersonations of celebrities such as Howard Cosell, Woody Allen and John Belushi -- "You could have called yesterday when your friend was home. But noooo! "

"We're doing very well," says producer/writer Hollie Davis, who launched the business in June with singer/writer Merle Medvene. "We got the idea after Merle moved to the West Coast last year and was complaining because nearly everyone in California has an answering machine. And I was complaining because people hang up on my answering machine. So we put our heads together and wrote some scripts."

"It's a lot like writing radio commercials," says Medvene. "There's a specific psychology involved in getting people to respond. A listener needs to be motivated to do something -- whether it's leaving a message on a machine or buying something from a store. You get the desired result by making it a personal and pleasant experience."

The tapes do work in reducing "hangs ups," says one Washington bachelor. "But now that I've stopped worrying about missing people's calls and people hanging up, my biggest worry is that no one will call at all."