The app at each end samples audio data from its own microphone and sends it off in chunks to the other end at the same time it takes the audio chunks received from the other end, stitches them back together and plays them out of its own headset or speaker. Once the call is accepted by the recipient – typically after the app has played a ringtone, popped up a message or both, and the recipient has opted in to the call – then the apps start exchanging network packets of audio data. When you make a Messenger call, for example, the app on your device – which could be a mobile phone, a laptop or even something like a smart TV – asks the Messenger cloud to locate the recipient’s device, and the apps at each end start negotiating to set up a call. Who’s listening?Īs you probably know, mobile voice messaging doesn’t rely on this “circuit switched” approach any more. in the circuit) at the same time, the additional electrical load on the shared circuit would prevent everyone’s ringers working and the exchange would not be able to put calls through to anyone. On a party line, where several homes were wired to a single connection, if too many households had their phones off the hook (i.e. On a single line connection, leaving the receiver off-hook prevented the circuit being used by anyone else, and therefore tied up a line in the exchange. You needed somewhere to store the receiver when you were no longer using it at the end of a call, so providing a place to hang it up that simultaneously disconnected the receiver from the circuit was a smart design decision – on the hook automatically meant out of circuit.Īctually disconnecting the receiver electrically from the circuit when not in use was important. Hooks weren’t a necessary part of the early telephone system, of course – in the exchange, calls were switched using jack plugs – but a gravity-operated switch that activated when the receiver was replaced or removed was a clever user interface choice.
Modern telephony is full of anachronisms.įor example, we still “dial” calls, and many phone apps still display the word “dialling” while they’re waiting for the person at the other end to pick up.īut when was the last time you saw, let alone used, a phone that actually had a dial?Īnd we still use idioms such as “ringing off the hook” to describe a day where we never seem to stop receiving calls, even though household phones haven’t actually had hooks since about 1912 and you’d probably have to go to a museum to see one.
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