James Martin/CNET

SAN FRANCISCO -- "Ahoy! Ahoy! Mr. Watson, are you there? Do you hear me?" So spoke Alexander Graham Bell to associate Thomas Watson, over a line stretching more than 3,400 miles, marking the first transcontinental telephone call.

"Yes, Mr. Bell, I hear you perfectly," Watson replied. "Do you hear me well?"

It was 100 years ago -- on January 25, 1915 -- that those words were the first to be spoken from coast to coast, a technological milestone that spurred a century of telecommunications innovation.

"The first transcontinental phone call was not only a breakthrough for AT&T, it was a key milestone in our nations's rich history of innovation," said Ken McNeely, president of AT&T California.

The four original phones used to make the call rarely come out of the AT&T archives for a public viewing. This past week, however, they were unveiled, and they will be on display at the California Historical Society in San Francisco as part of its "City Rising: San Francisco and the 1915 World Fair" exhibition at 678 Mission Street, which officially opens on February 22.

The completion of the transcontinental line in advance of the 1915 World's Fair was a significant engineering accomplishment. The stretch from Denver to San Francisco, in particular, was difficult to traverse and posed unique engineering challenges as the lines crossed the Rocky Mountains and vast undeveloped stretches of Nevada and Utah.

In 1908, the president of the American Telephone and Telegraph Company (now AT&T), Theodore Vail, had set a goal of building out the telecommunications infrastructure to create a transcontinental line. There was only one problem: at the time, the technology to achieve that goal did not exist.

AT&T began to study the amplification of electrical signals, and eventually, with the help of inventor Lee de Forest, developed vacuum tube amplifiers -- the key innovation that made it possible to extend the New York-Denver circuit the rest of the way to San Francisco.

That historic first call was initiated by Bell in New York City and included Vail, US president Woodrow Wilson in the White House and Watson in San Francisco.

"We are talking over 3,400 miles as easily and clearly as we talked over two miles 38 years ago," Bell said in the historic call.

Bell's first-ever phone call of any type had taken place in March 1876.

In the early part of the 20th century, communications from the East Coast of the US to the West Coast still took days. In 1914, travel by steamship through the Panama Canal took 16 days. By 1915, travel by transcontinental railroad had dropped to 90 hours.

But AT&T had now reduced the time for cross-country communications down to just 1/15th of a second.

The transcontinental service was officially opened that same evening at 9:01 p.m. The first commercial call was made later that night from a man in San Francisco who called his mother.

A three-minute coast-to-coast call cost $20.70 -- nearly $400 in today's money.