The largest and most complex round of labor negotiations this year will come to a head this week as the Bell System and the Communications Workers of America try to agree on a contract to see them through the upheavals of the impending break-up of the telephone company.

The union represents more than half a million workers in what is now the nation's largest corporation. In keeping with its tradition of no contract, no work, if no agreement is reached by midnight Saturday the union has vowed to strike, disrupting phone services.

Both sides have expressed cautious optimism that no walkout will be necessary. But the bargaining at the Mayflower Hotel which started May 19, reflecting a general confusion and anxiety over the phone system's future, is said to be moving more slowly than usual as a new multiplicity of voices emerges.

"The company has been much slower to offer specific proposals. There's no wage package yet," said CWA President Glenn Watts. "Both sides will be under pretty heavy pressure" as the strike deadline approaches.

"With divestiture, it's a whole new world," said Charles Dynes, spokesman for the American Telephone and Telegraph Co. (AT&T). "We don't have a package together yet, but I think we have plenty of time to fit the pieces together."

Under a court-ordered divestiture, AT&T will break into eight parts next January. Its 22 local operating companies will spin off into seven independent holding companies. AT&T will keep Bell Labs, Western Electric (equipment manufacturing), Long Lines and its new marketing subsidiary, temporarily dubbed American Bell Inc.

As the economic and institutional ground shifts beneath their feet, some Bell employes have been called on to choose whether to stay with their present company or move to the new marketing arm, gambling on which will prosper in the newly competitive atmosphere. Some jobs are being downgraded, some workers are being asked to move to new locations and some are being laid off.

"You would not believe the upheavals. People are on the edge of their teeth," said Pat Meckle, a 30-year veteran overseas operator and an officer of a CWA local in New York City. As New York's International Operating Center, which once employed 800 workers, is phased out, she said, "people have been at each others' throats" to get the 150 jobs that will remain.

As a result of such tensions, the union is putting less emphasis than in the past on big wage increases to keep up with inflation, Watts said, and much more on what he calls "employment security" to protect workers against the shocks of rapidly changing technology as well as of divestiture and economic shifts.

"Our members are feeling frustration and anxiety about the future," Watts said.

For those who must be laid off, the CWA is asking the company for "as much advance notice as possible," and for assistance with retraining, relocation expenses and other "bridging services." The union has also suggested "retreat rights" for those who choose, during divestiture, to go with a new entity that later lays them off, so that they can return to their previous jobs.

The union is pressing for more say in the development of new technology, and training and retraining programs to be funded by the company and jointly administered. It wants management to retrain "obsolete" employes to do new tasks rather than lay them off.

The union also wants AT&T to stop contracting work out.

The 19-city average for a skilled maintenance technician is $536.84 per week ($546 in Washington), Dynes said, and for a telephone operator $372.97 ($379.50 in Washington).