The nation's largest strike in three years ended yesterday when American Telephone & Telegraph Co. and the Communications Workers of America reached a tentative agreement that will send 155,000 telephone operators, technicians, clerical workers, factory workers and sales personnel back to work today after a 26-day walkout.

The settlement ends a turbulent month for AT&T in which supervisors manned telephones during 72-hour workweeks, 5,000 temporary workers filled in on switchboards, thousands of business phone-system installations were postponed and half of AT&T's factory production was shut.

Long-distance phone customers experienced only minimal disruption because the highly automated long-distance phone network handles 90 percent of the 33 million-call daily volume without operators. AT&T lost substantial revenue, however, because the volume of operator-assisted calls -- its most profitable service -- dropped during the strike.

"We are glad the strike is over and we will be glad to get everyone back on the job," AT&T spokesman Herb Linnen said. "Both sides win. Employes get a good labor contract that includes higher wages, improved medical, dental and pension benefits and, very importantly, an employment security program valued at $40 million."

"For AT&T, the agreement gives us flexibility" in creating new job titles and pay levels that help the company keep labor costs and prices in line with major competitors, he added.

"We got a lot of what we wanted and needed to settle this strike. We are very pleased," said Rozanne Weissman, the union's director of public affairs. "The total agreement . . . represents a major breakthrough in the telecommunications industry because of a far-reaching and innovative total employment security package and other gains" that met CWA's chief goals.

The strike ended following an all-night bargaining session in New Jersey Wednesday and a series of phone calls between top officials on both sides, settling issues including work schedules, job titles, severance pay, sales quotas and other disputes at AT&T's Information Systems, one of six AT&T divisions. Details were not disclosed.

Now CWA's 500 local units will vote on the contract. Final approval is expected by Aug. 4.

The strike was prolonged by the complexity of attempting to combine 23 regional labor contracts from the former Bell System into a single agreement. AT&T was conducting its first labor negotiations since the 1984 government-ordered breakup of the telephone monopoly.

Last week, AT&T and CWA reached tentative agreement on a nationwide contract that includes an 8 percent pay raise over three years, an 8 percent pension increase and a job-security plan that both sides hailed as a significant advance for high-technology employes who fear losing their jobs. Despite that agreement, CWA refused to end the strike until all six divisional contracts also were settled.

The three-year contract accomplishes a major AT&T objective by excluding payments for cost-of-living adjustments. But the contract does retain a COLA clause, which the union insisted on.

Wages for AT&T operators average about $21,300 a year, and for technicians about $31,700.

Wall Street analysts said that AT&T achieved many of its basic bargaining goals in negotiating moderate wage increases, no COLA payments and creation of a lower-paid job of "wire-puller" with a weekly salary starting at $240. AT&T was forced to use technicians making more than $600 a week to perform relatively simple tasks, and said this resulted in highly inflated installation costs.

The union also achieved many of its key objectives, including a greatly expanded job-security program highlighted by a $21 million fund to retrain workers for jobs within AT&T, or elsewhere in the event of layoffs. AT&T also agreed to expand the rights of employes to transfer to new jobs within the company, and will increase severance pay and payments to induce older workers to retire early.

The strike, the nation's largest since the 1983 AT&T strike by 500,000 CWA members, has created bad blood between that union and the International Brotherhood of Electrical Workers, which settled with AT&T on May 31 without striking, and whose 41,000 AT&T members remained at work at 12 factories. The IBEW settlement undermined CWA's clout during the strike and weakened the larger union's bargaining position, according to sources.