Some delays in operator-assisted and directory assistance calls were reported throughout the nation yesterday, but telephone service was largely unaffected as 675,000 unionized employes of American Telephone & Telegraph Co. went out on strike.

Instead of the voices of operators, customers often found busy signals or recordings that reported delays due to a "work stoppage."

However, representatives of the company and the unions agreed that the first real test will occur today as supervisory and management employes have to deal with the onslaught of business calls.

R. Webster Chamberlin, spokesman for the local Chesapeake & Potomac Telephone Co., said the company will rely on its 2,500 management and supervisory personnel in the District and its Virginia and Maryland suburbs to perform services normally carried out by 11,500 unionized employes.

He predicted that delays on operator-assisted or directory assistance calls will be minimal, probably less than a minute. The company will continue to perform repair and installation work but priority will be given to emergencies, such as homes or businesses without phone service, or to breakdowns at essential agencies such as hospitals, police staions and fire departments.

Contract negotiations for a new three-year contract between the company and the three unions, the Communications Workers of America, the International Brotherhood of Electrical Workers and the Telecommunications International Union, broke down Saturday over wages and job security.

There were no negotiations reported yesterday, although both sides said they expect talks to resume soon.

Union members walked off their jobs yesterday as the old contract expired at 12:01 a.m. local times, and picket lines were set up at locations around the country, including several in the Washington area. The last national strike by phone workers was in 1971 and lasted a week. There was a 72-day strike in 1955.

CWA President Glenn E. Watts said yesterday that he does not foresee a quick end to the strike.

"I would not expect a settlement in the next day or two," Watts said in a television interview. "On the other hand, we are very hopeful it will not be a long strike. But we are prepared for a long strike if necessary."

Watts said the union is holding to its demand that AT&T more than double its last offer on the bargaining table, which called for raises ranging up to 3.5 percent and included a cost-of-living formula identical to that in the old contract.

"We're talking about a settlement which will share the increased productivity in our industry and also keep our workers' cost of living up to date," he said.

The phone negotiations are the largest labor negotiations scheduled for this year. In a number of major talks in the past year, unions in such industries as auto manufacturing, trucking and steel settled for small gains and even agreed to "give-backs."

But the CWA argues that those were troubled industries. CWA officials, noting that AT&T had a $7.2 billion profit last year, said the company can afford to give its workers a better contract.

AT&T Board Chairman Charles Brown said yesterday that the company will not agree to the raises advocated by the union and is prepared for a long strike.

Brown said he does not anticipate major service problems. "Installation problems occur, "but I think we can handle the service job all right."

The bargaining has been made more complex by a court-ordered break-up of AT&T by the end of the year. As a result of the break-up, AT&T will difest itself of its 22 local operating companies, which will be reformed into seven independent regional companies.

"I think divestiture has caused complexities in bargaining, and there are a lot more issues than economics, I assure you," Brown said yesterday.

In the District of Columbia yesterday, Thomasine Jackson, a shop steward for Local 2300 of the CWA, said she doubts C&P's subsitute operators will be able to handle the deluge of calls on a normal business day. "Wait until all those government and business people get in and make calls," Jackson said.

The C&P management and supervisory employes will be required to work long hours as operators, phone installers, repairmen and at a variety of other tasks.

Some are being asked to perform their regular eight-hour assignments and then fill in on another job for three additional hours. Others will work 10-hour shifts at the substitute jobs.

Robert Sohl, 39, a member of C&P's marketing staff, normally helps shape corporate growth and development.

But yesterday, Sohl, a 17-year C&P veteran, slipped on a thin plastic headset, plugged it into a green console equipped with dozens of buttons and flashing lights, and became a telephone operator.

"You get nervous, there's no question about it," said Sohl. "As soon as you plug in, the calls start coming in. You don't get a chance to breathe."

Sohl said he "stumbled through" his first call twice before successfuly completing it. He said the caller, a young girl telephoning home collect, was patient while he fumbled for the right code to punch.

Sohl and scores of other C&P management employes were ordered to work at 6 a.m. yesterday. They were given a two-hour crash course on how to be operators and then went to work.

"I'd never seen one of these consoles until 6 o'clock this morning," said Thomas Krussman, a C&P account executive who worked as an operator yesterday.

Krussman and other employes working as operators said callers generally were patient, particularly when they realized there was a strike.

While Sohl, Krussman and others grappled with their new jobs, many of the regular operators were picketing outside the C&P office at 725 13th St. NW.

"When you put all this time in with the company, you want something back for it," said Sheila Fields, a C&P operator for almost 15 years. She said the striking operators' main concern is job security, particularly in the face of rapidly changing telephone technology that permits more and more calls to be placed without an operator.

A longtime switchboard operator at a private firm said yesterday that while the stand-in operators got the job done, she missed the regular operators.

"After you deal with them a lot, you get to know the voices," she said. "Today, the voices were all different."