By the early 1870s the telegraph had made instant communication possible for half a century, but it carried only dots and dashes. A crowded field of inventors was working on the "harmonic telegraph" — a way to send several messages over one wire at once — and in that race some began to realise that speech itself might be transmitted. The Florentine immigrant Antonio Meucci had been working in New York during the 1860s on what he called a "teletrofono" for transmitting voice, but could not afford the ten-dollar fee to renew his 1871 caveat. Alexander Graham Bell, an Edinburgh-born elocution teacher who taught the deaf, arrived at a similar discovery in Boston while experimenting with an electrical harmonic telegraph.

On 14 February 1876 Bell's lawyer and Elisha Gray's lawyer filed at the U.S. Patent Office within hours of each other — Bell a full application, Gray a caveat describing a liquid-transmitter telephone. The patent issued to Bell on 7 March. On 10 March 1876, Bell called from one room to Watson in the next: "Mr. Watson, come here — I want to see you." The working device used precisely the liquid transmitter that Gray had described in his caveat, an overlap that has fed one of the most contested patent disputes in history. In 2002 the U.S. House of Representatives, in Resolution 269, formally recognised Meucci's role in the development of the telephone, though it did not invalidate Bell's patent.

Commercial consequences came quickly. The Bell Telephone Company was founded in 1877; in 1878 the first telephone exchange opened in New Haven with 21 subscribers. In 1885 the American Telephone and Telegraph Company (AT&T) was founded to operate long-distance lines and soon absorbed its parent. By 1915 the first transcontinental call ran from New York to San Francisco. Unlike the telegraph, the telephone needed no trained code operator: user connected directly to user, voice to voice. It lifted ordinary communication out of an expert layer.

The significance of the telephone lay less in the volume of information it carried than in the way it personally collapsed distance. Work, family, and politics reorganised at once: a city centre could coordinate suburban production, newspapers could gather news in hours, emergency services and disaster coordination became possible. By the end of the twentieth century there were more than a billion fixed lines worldwide; in the early twenty-first the mobile phone surpassed that globally. The Meucci–Bell–Gray triangle also illustrates a recurring feature of modern technology: an invention is never the moment of a single mind. When the surrounding conditions ripen, several thinkers reach the same threshold at very nearly the same time.