How did vacuum tubes contribute to long-distance telephone communication?

Vacuum tubes, specifically the Audion triode invented by Lee de Forest, were essential to the development of long-distance telephone communication by providing a reliable means of signal amplification.12

Before the vacuum tube, telephone signals traveling over long distances suffered from significant attenuation, or loss of strength, which could not be adequately addressed by mechanical amplifiers or loading coils alone. De Forest’s 1912 innovation of "cascading" multiple Audion tubes allowed for the enormous amplification of weak signals by feeding the output of one tube into the grid of the next.21

The American Telephone & Telegraph Company utilized these vacuum tubes as critical components in repeater circuits, which boosted signals along the line. This technology enabled the first transcontinental telephone line between New York City and San Francisco in 1915, marking a major milestone in global connectivity.12