Who called Hawaii on the office phone and shouldn't do it again?

When you're not looking, they slip into the office to call an aunt in Hawaii, ring up a friend in Chicago, or get the latest on the weather. Meanwhile, your phone bill doubles and the boss's eyebrows are furrowed at you suspiciously.

Phone abuse is a growing problem in many businesses, adding as much as 40 percent to the corporate phone bill. Nationwide that costs business about $4 billion a year, says Harry Newton, a telecommunications expert in New York.

In 1978, the federal government was the victim of 798,000 unauthorized long-distance phone calls. Looking into this problem, New York's General Services Agency discovered it (GSA) was losing $3,000 dollars a month when their l5,000 employees made 50,000 unauthorized calls to phone services like dial-a-joke, dial-a-prayer, and off-track betting. The culprits included cleaning help, truck drivers, secretaries, and top-level management.

Employers, fed up with excessive phone bills, are seeking ways to combat the problem. Their solutions range from simple to complex:

* Plastic locks. Tele Vault Inc., a New York firm, recently started manufacturing plastic locks for push-button phones - not particularly exciting, but it works. The $12.95 lock clamps over two or all buttons with the turn of a key.

''It's the ultimate in cost control,'' says Mr. Newton, who recommends it to small law firms, restaurants, nightclubs, and other businesses with heavy public traffic.

At Johnson & Johnson, product manager John Sullivan purchased a lock for his own phone. His monthly phone bills often listed 75 calls, only a couple of which were his. By using the lock, he reduced local calls to two, and eliminated toll calls.

''The fact that locks are visible deters most people,'' says Mary Schaffer, of the Pentagon Library, another user of the Tele Vault lock.

Since introducing the project last May, sales have risen 45 percent a month for Tele Vault - about 5,000 more locks being sold each month.

Robert Parker, telecommunications manager at Boston's Prudential Insurance Co., is a bit more skeptical about the new lock. ''I don't think the device would pay for itself,'' he says. With 800 phones, it would work out to be $10, 360.

* Computer blocking systems. Better suited for large offices, these systems can block access to long-distance and local calling, disconnect lines to the popular ''dial-it'' services, and provide in-system tapes of weather reports or the time. They can even play messages warning employees they're calling a restricted number, says Richard Kauffman, former president of the National Society of Telecommunications Consultants.

Five years ago, the Kellwood Company, a clothing manufacturer for Sears, purchased a computer blocking system from Data Point Corporation. The system cost $65,000 and serves 600 users (people and computers). With the system, monthly toll calls dropped from 30,000 to 23,000 ($125,000 savings a year), and the average length of calls dropped from seven to five minutes, says Kerry Stewart, manager of Kellwood's telecommunications. The same system today costs about $200,000 dollars, says Stewart.

* Computer tracking systems. For $10,000 to $15,000, this kind of system, complete with printouts, can be attached to existing phone networks. They determine the origin, time, date, length, and costs of each call. Printouts circulate among employees who are then supposed to circle and pay for personal calls. Tracking is a helpful way of spotting evening or weekend violations, but less meaningful in its record of the myriad of calls during the work day.

Massachusetts General Hospital, with 5,511 telephones, sliced 38 percent off their $40,000 dollar monthly toll bill ($15,000 savings) by renting one of Bell Telephone's $500-a-month tracking units, says Evelyn Olschewski, telecommunications manager.

No matter which method is used, Richard Kauffman thinks just letting abusers know they're being watched is enough to cut back phone crime. For businesses on a tight budget he even thinks this will work: Construct an empty, black cardboard box with a lid. Label it ''Telecommunication Accounting System,'' and place it in a visible spot in the office. It reduces phone bills by 10 percent when employees suddenly become cautious, he says.

None of these methods is ideal. Locks keep others from abusing a phone, but not the key holder himself; computers can block lines on one phone while the phone next to it must remain completely unrestricted; printouts locate abuse, but not always abusers.

''Cutting down phone abuse is a very complex thing,'' says Mr. Newton. It is not something you can correct overnight. ''People often abuse phones because they're bored,'' he says. That becomes more of a sociological concern. Others look upon phones as a fringe benefit like company health insurance, retirement plans, or workman's compensation.