The dial tone is another example of something that will disappear with the next generation. I found this interesting, though.
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?’ ”