Striking New York Telephone Co. employes returned to work yesterday after being assured that their jobs are protected in a tentiative, new three-year contract with the Bell System.

The puzzling 16-hour walkout by Local 1101 of the Comunications Workers of America began at 12:01 a.m. yesterday several hours after negotiators for the national union approved the multibillion-dollar pact with Bell.

Company and national union officials said the strike had no significant effect on telecommunications work being done in New York City's Madison Square Garden for the opening today of the Democratic National Convention. s

"All of the necessary telephone work will be completed" by the time convention opens at 4 p.m., a New York Telephone spokesman said.

Spokesmen for the national union and the company expressed surprise yesterday morning when they learned of the strike by CWA's 33,000 workers in New York state.

"We don't really know what caused it," said a baffled CWA spokesman here.

"If I could tell you why they went out, I wouldn't be sitting here in this office. I'd be out on the golf course," said a New York Telephone Co. spokesman.

The closest anyone could come to a reason for the walkout was concern over job security. Local 1101 represents terminal ware (phones and other telephonic equipment) installers, cable splicers and other service workers whose jobs are being eliminated or "downgraded" through automation and technology.

"We just wanted to be sure that if someone invents a computer where all you have to do is type in a number to automatically repair something, we wouldn't lose our current pay or our jobs," said John Kelly, editor of the Local 1101 newspaper, "The Generaltor."

Union leaders and company officials said Saturday night, when the new contract was announced, it contains the strongest "job security" provisions of any labor agreement with Bell.

Embarrassed CWA officials started early yesterday morning trying to get that message across to their counterparts in New York. By noon, they said they had succeeded.

The new contract would protect of the life of the agreement, the pay of workers with 15 years' service who are threatened with "job downgrading through technological change."

Under the new pact, the company must notify the union before changing job titles. Any disputed job title change would be subject to arbitration.

Dollar improvements in pension benefits also are contained in the new agreement. But, most important, according to union leaders, pension pay penalties have been eliminated for workers with 30 years who retire before they are 65.

CWA President Glenn E. Watts, whose union represents 525,000 Bell workers, said Saturday night that the total value of the new contract is $5 billion over three years. But CWA officials revised that figure yesterday to $6 billion when wages and benefits for workers at the Western Electric Co., Bell's manufacturing arm, are included.

The new contract also won tentative approval yesterday from the international Brotherhood of Electrical Workers and the Telecommunications International Union which have 176,000 members in the Bell System.

Rank-and-file voting on the new contract will take place in about a week after all local union-company talks, including negotations involving 31,700 CWA members at the Chesapeake and Potomac Telephone Co., are completed.