Modern telephony is full of anachronisms.
For 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.
But when was the last time you saw, let alone used, a phone that actually had a dial?
And 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.
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.
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.
Actually disconnecting the receiver electrically from the circuit when not in use was important. 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. 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. 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.
As you probably know, mobile voice messaging doesn’t rely on this “circuit switched” approach any more.
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.
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.
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.
If the network is slow or unreliable, the app typically won’t drop the call, but will do its best to carry on anyway, either by leaving silent gaps in the audio, or by guessing in the case of short outages (that’s typically what is happening on an internet voice call when you hear a sound rrrrrrrepee-ee-ee-eated unnaturally), or by falling back to lower, scratchier quality.
In other words, there is no actual circuit that gets switched on or off between two internet phones, like there is between two old-school landlines connected to the same exchange.
Likewise, if the app has a mute button, it doesn’t work by disconnecting the microphone in your device electrically.
The apps at each end decide, based on data sent and received in chunks over the network, when to initiate a connection with a view to establishing a call; when to ring to signal an incoming call; when it’s OK to start recording and relaying sound; when to mute the call; and when to stop exchanging data and therefore “hang up” the call and to disconnect the virtual voice circuit.