Do you need a separate phone line to install a fax machine? A separate phone line is the best way to connect a fax machine, but the cost can be high -- up to $65 a month and a $50-plus installation charge. The best alternative, when available, is custom ringing, which assigns two phone numbers that generate distinctive rings to a single phone line. The cost is about $5 a month, plus an installation charge of less than $10. Lynx Automation Inc.'s $79 Ring Director (P.O. Box 99068, Seattle WA 98199, 206-285-1754) distinguishes the rings and routes the call to your phone or fax machine.

You can use a single ordinary phone line for both voice and fax, but there are limitations. For one, you should remove call waiting and other services that might interrupt fax messages. You have several choices for switching incoming voice and fax calls. Some are built into fax machines; others come boxed separately and cost $150 to $400.

Manual switch. The most awkward method is built into most fax machines. You answer calls with the fax machine's phone and press a button to turn on the fax. To receive faxes, you must always be at the machine, giving up much of its convenience.

Automatic attendant. Rainier Technologies Corp.'s TB-201 switch (17950 N.E. 65 St., Redmond WA 98052; 206-881-5151) answers calls with a recorded message and asks the caller to dial a digit to connect to a voice phone; callers who do nothing are automatically connected to the fax machine.

Active answer switch. Many voice/fax switches from Command Communications (10800 E. Bethany Dr., Suite 185, Aurora CO 80014; 303-750-6434), Multi-Link Inc. (1391 Leestown Rd., Lexington KY 40508, 606-233-0223), and others always answer the call on the first ring. The switch monitors for the presence of a fax CNG (CalliNG) signal (" beep-pause- beep"); if it finds the signal, it switches the call to the fax machine. Otherwise, the call goes to your phone. You or the caller can switch to a fax machine at any time by dialing a code.

Passive monitor switch. These switches, made by VSI Telecommunications Inc. (9329 Douglas Dr., Riverside CA 92503; 800-999-8232), Lynx Automation, and others, do not answer the phone; instead, you or an answering machine picks up the call. The passive switch listens for a CNG signal or waits for someone to dial a code before switching to the fax.

A few units let you alternate between active and passive operation. For fax calls without CNG, you can give callers the switching code in an answering-machine message, but the call will be dropped if a fax caller simply dials your number and walks away from the fax machine. -- Cary Lu