Executive Travel : Airline Phone Service Is Taking Off : Flying: Thousands have tapped into GTE technology that makes them reachable in the air.

Business travelers, it seems, really do want the seat-back telephone to ring with a call from the ground while they’re cruising at 33,000 feet.

In the first four months after GTE Airfone Inc. began offering the service on a few airlines, nearly 5,500 calls have been beamed up to passengers, GTE said. And 81,000 passengers have taken the steps necessary to make themselves reachable by phone while flying.

Phone-phobic travelers who would rather sleep, read or play a laptop computer game need not worry: The technology allows passengers to refuse calls with the press of a button or block all calls by not registering their seat numbers when they board.

GTE’s ground-to-air telephone service is now available on about 600 planes, according to spokeswoman Laura Littel. By mid-spring, most United Airlines planes will be equipped, she says. Other companies are offering their own versions.

GTE’s service was initiated in October, mainly on the USAir and Delta East Coast shuttles; United transcontinental flights between New York, Los Angeles and San Francisco; some non-shuttle Delta flights, and TWA and Reno Air. Aeromexico is being added.

Passengers who want to use the service must get an “aircall” number, which is free and can be obtained by calling (800) AIRFONE on the ground or *0-3-9 from an Airfone seat-back phone. The numbers are then sent out by mail.

To receive an Airfone call, passengers must pick up the seat-back phone, press 2, enter their seat number and drag their aircall card through the phone to activate it.

GTE charges a $2 fee to register the seat number, though that is being waived in a promotion during March.

To reach a passenger from the ground, a caller must know the party’s aircall number, then dial (800) AIRFONE, enter the aircall number, enter their own phone number and hang up.

A radio-based system on the ground sends a signal to the seat-back phone. Typically, a plane has one such phone for every three passengers. An LCD screen on the back of the phone lights up to indicate that a call is coming in for, say, passenger in seat 21B.

The passenger picks up the phone and drags his or her credit card through the slot to set up payment. The screen then displays the number of the telephone on the ground from which the call is being made and asks if the passenger wants to accept it.

If the passenger accepts the call, the party on the ground is reconnected. The whole process from the first call on the ground to link-up can take less than a minute, Littel says.

Setting up a call costs $2.50 and the rate for the call itself is $2.50 per minute. Through March, the company is offering a 30% discount on those charges.

The digital technology involved also allows passengers to send faxes or download files from certain laptop computers. The charges for data or fax calls are $1.50 for setup and $1.50 per minute. Users can also access the Internet, receive data or a fax and use other on-line services.

Littel says the system is being fine-tuned over land initially but should eventually be available on transoceanic flights through the use of satellites.