NEW YORK -- Prodigy Service Co. said Tuesday it was joining three regional Bell operating companies to become the first online company to offer faster access to its services and to the Internet computer network by using digital phone lines.

A spokeswoman said Prodigy, a joint-venture between International Business Machines Corp. and Sears Roebuck and Co., will be the first online service to offer its customers high-speed digital phone service.

A formal announcement was planned for Wednesday, she said.

Analysts said they expected Prodigy to announce that it was joining BellSouth Corp., Nynex Corp. and Pacific Bell to give Prodigy customers access to what is known as ISDN, or integrated services digital network.

The analysts said Prodigy was again moving ahead of its rivals, who also have made plans to provide ISDN service.

A spokeswoman for Nynex in White Plains, N.Y., confirmed that it was working with Prodigy. She said the service would be available first in the Boston area.

Officials at Atlanta-based BellSouth and at Pacific Bell, a San Francisco-based unit of Pacific Telesis Group, did not return calls. But industry sources confirmed that those three Baby Bells would be involved in the Prodigy announcement.

Earlier this month, Pacific Bell said it had launched a major initiative to make ISDN accessible for mainstream computer users and online services.

Microsoft Corp. recently invested in a private company, UUNET, to offer ISDN access on the Microsoft Network, which will be launched in August with the company's Windows/95 upgrade of its popular operating software.

Prodigy said its service running over ISDN lines will be four times as fast as a modem running at 14,400 baud.

``It's twice as fast as 28.8 (kilobytes per second). It flies,'' said a Prodigy spokeswoman. She said Prodigy's World Wide Web browser for the Internet ``just screams on this.''

Prodigy hopes to sign deals soon with the other four regional Bell operating companies, so it can offer its ISDN service to all regions of the country.

While Prodigy will not charge extra for the service, users will have to buy extra equipment, such as an ISDN modem and a communications port card for their computer.

IBM has developed a consumer-grade ISDN modem called the Wave Runner, which it will unveil as part of the package Prodigy will be announcing Wednesday, a source said.

The cost for the ISDN service will be charged by the telephone company. In New York, Nynex charges residents about $24 to $34 a month for basic telephone and ISDN service, plus separate installation charges for the line.

Prodigy will begin offering the service in May.

Analysts said Prodigy's ISDN service would likely be initially attractive just to Internet junkies, but that as the cost declines, it will attract average users.

``The bottom line is ISDN helps people with a high demand for fast graphics, video, and other high bandwidth uses,'' said Peter Krasilovsky, a senior analyst with Arlen Communications in Bethesda, Md. ``As more people get onto the World Wide Web, the value of ISDN will become apparent.''

A Prodigy spokeswoman said that the ISDN service comes at a perfect time because of the great interest in its recently introduced World Wide Web browser for the Internet.

The online service was the first to introduce a Web browser, getting a jump on its main rivals, H&R Block Inc.'s CompuServe and America Online Inc., and as a result, it has gained some new respect in the industry.

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