American Telephone & Telegraph Co.'s Western Electric division yesterday announced plans to phase out its oldest and largest manufacturing plant, the 80-year-old Hawthorne Works in Cicero, Ill.

The plant, which currently employs 4,200 workers, will be phased out over three years. In recent years, the Hawthorne Works has been made increasingly obsolete by modern production techniques.

Although the company did not set a specific date for the closing, a Hawthorne Works spokeswoman said the plant would probably close its gates just outside the Chicago city limits sometime in 1986.

A spokeswoman for the plant said that half of the workforce, which averages 49 years of age, would be eligible for pensions by 1986; attempts will be made to place the other workers at Western Electric facilities in the Chicago area or elsewhere in the nation. The company will also attempt to set up a retraining program for workers affected by the closing.

The plant, which once made all the telephone sets and switching equipment used in the United States, now makes cable, circuit boards and capacitors. At its peak, during World War II, the plant employed 48,000 workers; in 1970, 23,364 persons were on the Hawthorne Works' employment rolls. The number has been dropping steadily ever since, spurring repeated rumors that the plant would eventually be closed.

The Hawthorne Works spokeswoman said it was not yet known what would be done with the plant's facilities, which sprawl over 141 acres in an industrial neighborhood. Over the past several years, Western Electric has been demolishing major portions of the plant's brick superstructure totaling 1.4 million square feet and moving most operations into a more modern building on the site.

The spokeswoman said Western Electric officials would be meeting with state and local officials to discuss possible uses for the plant. Such facilities as the factory's new waste treatment plant may be useable by other companies or ventures in the area, she said.