Someday, telephone customers will tap directly into phone company computers for directory assistance, greatly speeding the process but eliminating the jobs of thousands of women and men who now provide the service.

New jobs, however, may open up for them as their employer, American Telephone & Telegraph Co., competes against International Business Machines Corp. and other firms in new high-technology ventures that were off-limits to AT&T before the breakup last year of the Bell System.

These and similar revolutionary changes in the work force, in technology and in corporate structure are confronting the Communications Workers of America, AFL-CIO, as it enters a new era under an aggressive and politically active new leader. Technological advances are eliminating some jobs and creating others, while workers struggle to retain pay, benefits -- and their jobs -- in the fiercely competitive deregulated environment that pits giant corporations against each other.

Yesterday the CWA, whose 650,000 members make it the world's largest communications union, installed its third president in five decades, Morton Bahr, 59, a CWA vice president from New York who was unopposed in his campaign to replace the retiring Glenn E. Watts.

In his acceptance speech at the union's convention in San Francisco, Bahr lost no time in promising new union organizing efforts, including a bold attack on nonunion IBM. The CWA, he said, will launch a "worldwide coordinated organizing campaign against IBM" involving U.S. and foreign unions that deal with the global corporation.

Bahr added that he would strongly resist "give-backs" demanded by employers and would strengthen CWA's political action effort for the 1986 and 1988 elections.

"Once-mighty unions are shells of their former greatness," he said, "The same can happen to CWA if we allow ourselves to go into intellectual retirement."

The transition from Watts to Bahr comes at a crucial time for the CWA. The union is facing its first nationwide bargaining next year in the brave new world of the post-divestiture telecommunications industry.

When AT&T enjoyed monopoly status, the CWA too benefited from a 100 percent unionized industry. But the union has lost more than 50,000 jobs in what was the Bell System, and more cuts are expected. The unionized regional telephone operating companies are frequently opening new nonunion subsidiaries. While the telecommunications industry burgeons, its work force is estimated to be about 35 percent unionized.

Among CWA's innovative job-saving efforts has been a $2 million "Call and Buy Union" advertising campaign aimed at persuading customers to stay with AT&T rather than transferring their service to new long-distance companies. And Bahr promises more of the same.

"Either we become dinosaurs, or we face our greatest challenge" by beefing up organizing efforts, Bahr said in an interview. Such efforts, he said, must be global because U.S. firms can engage in "tele-scabbing" by closing unionized operations here and using telecommunications technology to run nonunion data-processing and other operations overseas.

Bahr, who has been a key supporter of New York Gov. Mario M. Cuomo, is also likely to be heard increasingly in national Democratic politics. Bahr said he believes the AFL-CIO should again consider a preprimary presidential endorsement, despite the urging of Democratic National Committee Chairman Paul G. Kirk Jr. not to do so. "If we believe early endorsement is in the best interests of the country, we will likely do it again," Bahr said.

Bahr's emphasis on organizing and his aggressive and outspoken style will be a change from Watt's quieter and more intellectual approach, according to union officials.

Watts, 65, a member of the Democratic National Committee; the Trilateral Commission, a private, international business leaders group, and the executive council of the AFL-CIO, has been regarded as an innovative unionist who launched CWA studies of the future of U.S. industry and who supported new "quality of work-life" programs that encouraged labor-management cooperation with AT&T.

In Watts' 11-year tenure, the CWA was one of the fastest-growing unions, thanks to industry expansion in the 1970s and the union's push into the public sector. It represents more than 50,000 government employes, including New Jersey's state employes and many in Ohio and other states.

Watts said in an interview that after 44 years in CWA, starting as an installer with the Chesapeake & Potomac Telephone Co. here, he believes the union is well-positioned to grow. But he said he wanted to turn over the CWA to "strong, energetic people" who will "revitalize" organizing.