A nationwide telephone strike is "almost inevitable" this weekend, the leading negotiator for 700,000 unionized Bell System employees said yes-he called a "complete standstill."

Glenn E. Watts, president of the Communications Workers of America, said a strike would last for months and cause a "substantial disruption" of service.

But a company spokesman said chances for a settlement remain good. Even if a strike occurs, automated equipment, backed up by 200,000 non-striking supervisory employees, would provide virtually uninterrupted service to most customers, he said.

"The public wouldn't feel much impact for at least some time," said Charles Dynes, spokesman for the American Telephone & Telegraph Co. in Washington, where negotiations are being held.

Watts and Dynes agreed that dial telephone calls, including most long-distance calls, would not be immediately affected.

But Watts said the 4.8 million operator-assisted calls that are made daily would be disrupted, along with both business and residential telephone installations. Dynes said operator assisted calls and equipment installations would continue but with some delays. They also disagreed over whether supervisory workers are suficiently skilled to make repairs on sophisticated equipment.

The last telephone strike six years ago lasted two weeks, Watts said. A strike was narrowly averted three years ago when the current contract, which expires at midnight Saturday, was negotiated.

The strongly worded warning from Watts, whose union is the largest of three involved in the negotiations, appeared aimed at forcing an immediate resumption of talks that were recessed inconclusively Tuesday.

Early last evening, it was announced that talks would resume this morning between the company all three unions, raising speculation that AT&T may make a new contract offer. However, Dynes said, "I don't think it necessarily means anything more than they're talking."

At a press conference earlier in the day. Watts indicated bargaining teams were far apart on money and job security guarantees, which the unions are demanding in light of what Watts described as a 100,000 loss of jobs to machines since 1974.

"I can see no way to avoid a strike unless there is a dramatic breakthrough in the next few hours, which I candidly do not foresee," said Watts, adding that it would take a "miracle" to settle by Saturday.

In its first and last wage offer since contract talks started in May, AT&T proposed an increase of 18.3 per cent over three years. Weekly wages now range from $129 for a starting telephone operator in Birmingham, Ala., to $333.50 for experienced electrical and other skilled craft workers in New York City.

The three unions - CWA with 500,000 Bell System members, International Brotherhood of Electrical Workers with 120,000 and Telecommunications International Union with 65,000 - rejected the offer.

Watts said yesterday his members would not settle for anything substantially less than the money packages won by auto and steel unions the past year. These settlements amounted to roughly 30 per cent spread over three years, according to the Council on Wage and Price Stability.

On job security, which was also a key feature in the auto and steel settlements, Watts said the telephone unions want progress toward a shorter work year to spread jobs among more people, retraining and reassignment of job-threatened workers and supplementary pensions for early retirement. He said the company has "closed its eyes" to these ideas.

Despite Watts' pessimism, Bell spokesman Dynes said he was optimistic that a strike could be averted. "We're certainly willing to bargain," he said. "There's still two days."

But Watts said two days may not be enough. An agreement would have to be reached by "early on Saturday," he added, warning the company against any 11th-hour offers. A strike, once called, would "take weeks to wind down" and is likely to last "for months," Watts said.

Watts said he has notified President Carter, the Secretary of Labor, Federal Mediation and Conciliation Service and members of Congress that a strike is likely and told the company that the union is prepared to "bargain around the clock" through Saturday. Federal mediation efforts had not been requested as of yesterday morning, he said.