Phone systems which carry data and voices increasingly in vogue.

The U.S. General Services Administration's office in Long Beach is joining a growing cadre of Los Angeles area business offices experimenting with the next wave in telecommunications technology: Integrated Services Digital Network or ISDN.

The technology, which carries both a voice conversation and data transmission on one line, is considered as revolutionary as compact discs were to the record industry. In describing ISDN, a Pacific Bell brochure calls it "converting a telecommunications dirt road into a paved digital highway."

The General Services Administration, like all federal agencies, is under order by Congress to update its telecommunications system by the year 2000. The Long Beach GSA office is experimenting with ISDN lines installed by GTE, which allow two people on separate computers to talk on the phone and simultaneously edit the same document.

Data transmission is 50 times faster than the standard modem rate, according to Lennard Glogauer, area general manager of business applications for GTE.

Normal data transmission on a modem is at about 9,600 bits per second, which allows transmission of only one screen of information at a time. With ISDN, transmission is speeded to 56,000 bits per second, which allows entire files to be transmitted at a time.

ISDN is part of a trend toward multimedia applications, combining video images and telecommunications.

"Eventually, everybody will have ISDN," said Jim Diestel, Pacific Bell's project/product manager for ISDN.

Pacific Bell has been offering ISDN lines to customers in the L.A. County area since the beginning of this year, and now has about 50 businesses trying out the product. GTE only started offering ISDN last month.

Diestel said law firms are signing up for the service because of its high-speed access to on-line data bases.

The San Fernando Valley Board of Realtors is testing an application of ISDN called Realty Link, which packages multiple real estate listings on the computer instead of having clients comb through volumes and volumes of listing books.

Realty Link, sold through Pacific Bell, can call up all listings that fit a client's requirements, such as price, neighborhood and style. Customers can even take a model tour of the home by using their computers.

"It's automating the Sunday open house," said Cindy Donovan, a director in Pacific Bell's advanced products and services department in San Ramon.

ISDN is also being used in teleradiology -- transmitting X-rays for analysis. This will have a big impact in rural locations which don't have access to a radiologist. ISDN is also being used for downloading blueprints, transmitting images from checks and photos from accidents, and delivering letter-quality faxes. ISDN is also expected to lead a boom in telecommuting. And it's being considered a great equalizer for the handicapped.

Although ISDN technology is only offered to certain locations, it is expected be widely available in the next year.

Pacific Bell just announced an upgrade of its L.A.-metro area system to digital switches, which will give all area businesses access to ISDN by 1993, said Diestel. He said by 1995, Pacific Bell will offer ISDN to 95 percent of its customers statewide.

Meanwhile, GTE currently offers ISDN to customers in Walnut and parts of Long Beach, said Glogauer. Soon ISDN will be available in Thousand Oaks, Norwalk and Redondo Beach. By 1993, he said, it will be available in Santa Monica.

Companies are easing into the technology by setting up just a few ISDN lines, according to phone company officials. An ISDN line actually consists of three separate lines, two for voice conversations and one for data. The price of an ISDN line is about 20 percent more than that of a regular line, but it is cheaper than getting three different lines.

Currently Pacific Bell only offers ISDN to its Centrex customers -- customers who are hooked into Pacific Bell's network in lieu of having their own switching equipment -- at a cost ranging from $1.85 to $10.35 per line per month, plus an installation charge of between $100 and $150, said Diestel. He said Pacific Bell is now working on a tariff to offer ISDN to customers without Centrex for a comparable price.

GTE offers ISDN for about $5 more than a standard line per month.