“Remove the Receiver from the hook,” read a notice in The Norfolk Ledger-Dispatch in 1919, “and listen for a steady humming sound, known as the ‘Dial Tone.’ If you hear the dial tone, place your finger in the hole containing the first figure of the number to be called and pull the dial as far as it will go. . . .” (An accompanying article described the noise as being “like the buzzing of a bumble bee or other large insect.”)

Never before had an American telephone company employed this signal, says Roger Conklin, a telephone collector and historian. Until then, the leading telephone provider, the Bell System, relied on its corps of “hello girls” to connect lines at central switchboards so no dial tone was necessary. Other phone companies did use an automatic dial system. (According to the Western University historian Robert MacDougall, the providers of the “girl-less” lines promised that they never eavesdrop and are “never impudent and saucy.”) But they didn’t bother with a dial tone either. Their systems were simple enough that customers rarely suffered serious delays in making connections.

On the new exchanges in the Bell System, though, users were more likely to run into glitches: More efficient network systems were vulnerable to spikes in traffic. After the company finally adopted the automatic-dial technology in 1919 — a telephone-operator strike in Boston that year seems to have hastened the change of heart — customers sometimes had to wait for a connection. If they started dialing too soon, some pulses would be lost, and they would get a wrong number. By instructing people to hold for a dial tone, Bell System engineers solved that problem. The buzzing sound was also reassuring. “The dial tone is the equipment’s way of telling you that it’s ready to put through your call,” explained an instructional film called “Dial Comes to Town.” “It’s the same as an operator saying, ‘Number, please?’ ”