The Bell System and the Communications Workers of America reached a tentative contract settlement late yesterday, just hours before a nationwide telephone strike by 700,000 union workers was to begin.

CWA President Glenn E. Watts told reporters the agreement, which is subject to ratification by the rank and file, will provide an increase of 31 per cent in wages and fringe benefits over the three-year life of the settlement. The increase is comparable to those reached by the auto and steel unions.

The past, reached after and all-night bargaining session and threats of an "almost inevitable" strike by Watts, calls for wage increases of 7.8 per per cent in the first year, 7 per cent in the second and 6.7 per cent in the third, assuming cost-of-living increases based on 6 per cent inflation. Watts also said pensions would be increased between 25 and 30 per cent and new job security protection would be provited, including additional days off to spread jobs amongs more workers.

Still negotiating with the American Telephone & Telegraph Co. were two unions with smaller representation in the Bell System, the International Brotherhood of Electrical Workers and the Telecommunications International Union.

Before yesterday's setttlement, company and union officials had agreed that a strike would cause no disruption in dial telephone service, but Watts had warned Bell's 125 million customers to expect "substantial disruptions" in operator-assisted calls, installations and repairs.

Negotiations met through the night until 6:30 a.m. yesterday and continued through most of the day, encouraging speculation that progress was being made and a settlement was possible.

The Federal Mediation and Conciliation Service stepped into the talks late Friday after the CWA and the two smaller unions rejected a second company offer.

No details of the second offer were disclosed, but it reportedly narrowed the differences, which were so wide that on Thursday Watts claimed that a strike was "almost inevitable."

In its first offer last month, the company proposed a wage-and-benefit package representating an increase of 18.3 per cent over the three-years. Watts said the CWA, would not settle for less than the three-year, 30 per cent increase won over the past year by auto and steel unions, and said some progress would have to be made toward job security protections.

The highly computerized telephone system handles 485 million completed calls per day, more than 90 per cent of them without operator assistance.

A key question underlying the three months of telephone talks - a question with major implications for labor-management relations in the rapidly expanding communications industry - was whether the system is now so sophisticated that it is essentially strike-proof.

Work days lost through strikes in all industries dropped off during the first half of this year. But they appeared to be climbing this summer as 15,000 iron ore workers belonging to the United Steelworks struck in the upper Mildwest and as many as 70,000 United Mine Workers members, one-third of the unions membership, continued wildcat in the Appalachian coal fields. A full-fledged national coal strike is expected in December.

Wage agreements reached thus far this year generally have been lower than settlements won last year: 6 per cent over the life of a contract, compared with 6.4 per cent for 1976, according to the Bureau of Labor Statistics.

As of last year, communications workers - ranging from clerical employees to skilled electrical and installation personnel - received average hourly earnings of $6.77, according to the Council on Wage and Price Stability. Other industries, bargaining this year include coal ($8.10), steel ($7.84), construction ($7.81), railroads ($6.98) and retail foods ($6.40). Steel wages, under a contract negotiated in April, now exceed $8 an hour.

Weekly telephone pay reportedly ranges from $129 for starting operators in Birmingham, Ala., to $333.50 for experienced craft workers in New York City.

Job security was a dominant issue in this year's bargaining because the telephone system has lost 100,000 jobs since the 1974 contract was signed, with the unions attributed most of the loss to accelerating automation.

The CWA, which once welcomed automation as a boon to job expansion, now finds itself retrenching in the face of unanticipated consequences, union officials noted.

Although a strike in the Bell System was averted three years ago, brief walkouts of about a week's duration occurred in 1971 and 1974, with the earlier walkout lasting seven months in New York. A strike probably would have occurred in 1974 if the unions had taken a strike-authorization vote in advance of the contract expiration. This year they took the authorization vote.

As the nation's largest private employer, the Bell System roughly 1 per cent of every 100 workers in the country: 900,000 out of 90 millions. The current Bell System contract cover 23 regional companies, Bell Laboratories and Western Electric Co., the Bell manufacturing division [TEXT OMITTED FROM SOURCES]