The click in the middle of a conversation
Most people who have used a landline telephone know the sound: a brief tone, or a click, in the middle of a call, telling you that someone else is trying to reach you. Before call waiting, a person already on a call was simply busy. A second caller heard a busy signal and had to try again, and the person on the line never knew they had called.
Why it needed new switching
For early electromechanical exchanges, a line was either free or in use, and the exchange could not easily detect or handle a second call attempt to an occupied line. Making call waiting work meant the exchange had to do several things at once. It had to recognize that a second call had arrived for a busy line, send a tone to the subscriber without disturbing the existing conversation, listen for a signal that the subscriber wished to switch, hold one caller and connect the other, and allow switching back and forth.
These tasks suit electronic computers. They are logical decisions about the state of a line, and so call waiting is closely associated with the stored-program exchanges introduced from the mid-1960s. Along with call forwarding and three-way calling, it was marketed as part of a package of custom calling services. The services rolled out in stages over the late 1960s, the 1970s and later, and the date and availability depended on the country, the carrier and the type of exchange. Because the history is told in different ways by different sources, it is safest not to name a precise first day or place.
How it works
The sequence is generally the same in most versions.
- A subscriber is on a call and a second caller dials the same number.
- Instead of a busy signal, the second caller hears ringing, and the exchange sends a short tone to the subscriber, audible only to the subscriber, not to the other party in the conversation.
- The subscriber can ignore it, or press the switch hook or a flash button briefly. This hook flash signals the exchange to put the first caller on hold and connect the second.
- Another flash returns to the first caller. Each flash toggles between the two.
- If the subscriber hangs up, the exchange rings back so that the held caller can be answered.
Telephones with a flash key made the process easier. Without one, people had to tap the switch hook with the right timing, which could accidentally disconnect the call.
Etiquette and annoyance
Call waiting introduced a new social question: is it polite to interrupt one conversation for another? People disagreed. Some felt that putting a friend on hold to answer a different call was rude, while others considered it a necessary convenience, especially for businesses and for those who wanted to avoid missing urgent calls. Newspapers and etiquette columns discussed the question. Many users adopted conventions such as asking permission before switching, or checking quickly who was calling and returning to the first caller.
The dial-up problem
Call waiting had an unintended effect with home internet access. In the 1990s, during the early history of the internet for home users, many people connected through dial-up modems that used the ordinary telephone line. A call waiting tone arriving during a data session could corrupt the signal and disconnect the modem. This led telephone companies to offer a way to disable call waiting temporarily. In North America, dialing a prefix code before making the call turned the feature off for that call only. People who used the internet frequently also found other solutions, such as a second line, and the arrival of broadband made the issue less common.
Mobile phones
On a mobile phone, call waiting is part of the cellular network standard. A notification appears on the screen, often showing the second caller’s number or name, and the user can accept, decline or send the call to voicemail. Smartphones turned the hook flash into an on-screen button, and added options such as merging the calls into a conference. The tone is less intrusive than the old click because the caller’s details are visible.
For calls made over the internet, the idea carries over, but it is implemented in software. Apps may show a banner for an incoming call during an active call, and the user chooses how to respond.
A small feature with a long reach
Call waiting looks like a minor option, but it altered expectations in a lasting way. It made the telephone interruptible. A person was no longer simply busy or free, and the possibility of a waiting caller turned conversation into something that could be paused. That idea fed into the multitasking habits of messaging, notifications and calls that we take for granted.