When the phone rings in her Santa Monica apartment and the caller asks Sharon Johansen to "come over and see my new Doberman," she doesn't think it's a crank call. Miss Johansen is a dog trainer and she constantly gets calls from her pupils' owners. "Someone's always phoning to tell me about a ribbon one of my dogs has won or a litter she's had. I don't have any kind of shop or office where people can bring their pets, so I go to them instead. I usually become friends with the owners as well as their dogs," explains 23-year-old Sharon. Her occupation grew out of frustration with the conventional restrictions of nine-to-fiveism. "I had an interesting desk job, too. Right after high school, I worked for Pierre Salinger's investment company. Knowing him was great, but I hated being cooped up in an office all the time. I also wanted something that would give me a chance to be my own boss, make my own hours. I've always loved animals - at first I considered working at the L.A. zoo. Then it came to me out of nowhere - dog training. I took a course to learn how it's done and just started out." She has built her business slowly and carefully, relying on word-of-mouth advertising to attract new prospects. "I prefer working that way, so when people call, they've already made up their minds to have their dogs trained and I don't have to sell them on the idea. Most of them also know what it costs. I charge $200 for a full course or $20 an hour. I know that sounds like a lot, but I don't work every day, so I had to think about that when I set my fee." Since the economics of Sharon's business restricts her clients to a relatively affluent group of dog owners, her house calls are made principally in Beverly Hills, Palos Verdes and other lush, meticulously gardened areas of Los Angeles. "That's a great part about my job. I've spent time in some of the most fabulous backyards in Los Angeles." Another is the free time it gives her to get involved in a new career - acting. Sharon has thought fancifully of being an actress for as long as she can remember, but "I never had the nerve to even audition for a part in high school. I'd always get as far as the door of the room where the drama club was holding tryouts. I'd see them all in there reading scripts - and I'd chicken out. So it never occurred to me that I'd have the nerve to actually go through with it. I think starting the dog training business did it for me. I proved to myself that I could do something that wasn't routine and make a living on my own. That gave me confidence. I'll need it, because when you're trying to make it as an actress, the disappointments can really bring you down." So far, Sharon has little reason for regret. She's already appeared in a number of television series, including The Name of the Game and Sarge, and in a Li'l Abner special, in which her conspicuous proportions (40-22-37) were well suited for the role of Appassionata Von Climax. Now Sharon has just completed shooting her first feature film, Your Three Minutes Are Up. "I play a kind of beach girl named Johansen. That's really wild because I get to use my own name and I couldn't be more of a beach girl in real life. My Santa Monica place is just a block away from the ocean and I spend all the time I can there. Everyone who's seen the first rushes says it's a terrific film. Right now I'm testing for a Love, American Style part - and a lot of other TV things. It just seems like everything's rushing in at once. And I love it." If her career maintains its present pace, Sharon may soon find herself with a Beverly Hills back yard of her own.