Although the Chesapeake & Potomac Telephone Co. transmit millions of conversations every day, there are relatively few words to be heard in its office nowdays.

In the company's old downtown office on 13th Street NW, large gray banks of electronic switching equipment click and whirr. The machines are tended most of the time by just three rather quiet technicians.

As C&P parent company, the giant American Telephone & Telegraph Co., and its 700,000 unionized employees head toward a nationwide strike deadline at midnight tonight, the workers and the highly automated machinery are the central point of conflict.

Although AT&T remains by far the largest private employer in thw world, the size of its work force has declined by 10 per cent over the past four years because transistorized electronic equipment is being introduced on an ever-widening scale.

As a result, job security has joined wages as a key issue in the current negotiations between AT&T and its three unions. The talks are being held in Washington at the Mayflower Hotel.

Yesterday, negotiators for the Communication Workers of America, which represents about [LINE ILLEGIBLE] offer but promised to bargain "around the clock if necessary" to try to resolve differences.

Two smaller unions - the International Union of Electrical Workers with about 120,000 members and the Telecommunications International Union (with 65,000) rejected the offer later in the day.

Terms of the proposal were not disclosed, but union negotiators indicated that it represented some progress toward a settlement. On Thursday, CWA president Glenn E. Watts warned that a nationwide strike was "almost inevitable."

Yesterday, leaders of CWA locals in the Washington area said picket signs had already been printed and distributed. A spokesman for C&P said that if a strike breaks out, service would continue without interruption because the system is so highly automated and management employees can fill in for striking workers.

"We'll certainly make an effort to provide service as usual," said Web Chamberlin, C&P's chief spokesman. "We will have to sacrifice to make the system work because of public wants and needs service."

Chamberlin added, however, that the longer a strike continues, the more company officials will have to "priotize" what they do, and the more likely it is that some services, such as new installations, will be delayed.

C&P Telephone actually is a group of four companies - all wholly owned by American Telephone & Telegraph - that serves about 7.5 million telephones in the District, Maryland, Virginia and West Virginia.

IN the Washington area, the company has 2,891,000 telephones - up almost 18 per cent since its last strike, which lasted about one week, in 1971. The average daily number of calls per phone here (8.29) has gone up by about 15 per cent over the same six years, the company said.

But the number of workers employed by the company has dropped by 19 per cent as new equipment has been installed to do their jobs.

All of the reduction in jobs has been achieved by attrition, Chamberlin said, with workers either quitting or retiring and no obe being fired.

Charles Sangmeister, staff representative for the CWA in the Washington area, said the trend had been "kind of frightening."

"It used to take 40 or 50 people to run one (switching) office," Sangmeister said. "Now it takes 10."

In C&P's 13th Street office, most afternoons there are just 25 operators on duty to handle all the calls that need operators - except directory assistance - from the 1,026,000 phones now installed in the city.

The operators work in a bright, carpeted room, which they have named Watergate, with each operator sitting at a desk rather than a long switchboard.

Although up to 35 operators can work in the room, company officials said the number is rarely needed.

In fact, overall, C&P employs just 1,458 operators in the Washington area, and almost half of them handle inquiries to directory assistance. In 1971, the number of operators here was 3,318.

The number of installers, switchmen and repairmen also has declined - though not so sharply - from 4,865 in 1971 to 4,132 today.

Much of their work has changed, Chamberlin said. In central offices where equipment is transistorized, a machine types out the location of faulty part, and a technician can replace it quickly with another one, rather than spend time tinkering with wires and circuits.

Telephone equipment in businesses and homes also has been made much simpler to repair, and the equipment is studier than earlier models, Chamberlin said, and breaks down less often.

All this means lower costs for the company and the public that pays for telephone service, company officials say, keeping down rates and increasing profits. Union officials, however, see it as a threat to workers and jobs.

"You can see the handwriting on the wall," the CWA's Sangmeister remarked. "Automation is doing quite a job on people."

Besides its 10,100 unionized workers, C&P Telephone has about 2,000 salaried management and supervisory employees in the Washington area. In addition, Western Electric, the manufacturing division at AT&T, employs about 1,600 unionized workers here many of them repairing equipment at a large plant in Arlington. AT&T Long Lines division has about 1,000 unionized wirkers here, designing and servicing long-distance telephone lines.

Sangmeister said the union doesn't want to force the company to use obsolete equipment, but it has proposed shortening the work-year with longer vacations as a way to spread jobs among more people and increasing pensions for workers who retire early.

So far, the company has not agreed to major changes in its vacation and pensoip plans, AT&T officials said.

In addition, the unions have asked for a 30 per cent wage increase over three years, while the company in its first offer made last month, proposed an 18.3 per cent increased over three years.