Hold works, with music for whoever is waiting.
Press hold on a desk phone and the other person hears music instead of silence.
What hold really is
When you press hold, your phone tells the system it has stopped listening. The system then does two things: it stops passing either person's voice to the other, and it plays music to whoever is waiting.
Upload your own music, or live with our built-in tone, which is pleasant for about nine seconds.
One continuous stream
The first version had a flaw you could hear. Going from voice to music, some phones went quiet for a second or dropped the start of the music.
The reason is that every source of sound has its own clock. Voice from the other party, music from a file and a tone made by the system each count time separately. A phone that suddenly gets audio that seems to have jumped in time may throw it away.
So everything the system sends to a caller now goes out through one place, which puts it all on a single continuous timeline and marks the moment the source changes. To the phone it is one stream from start to finish.
Why it matters later
This small piece of plumbing is what will let a caller hear a menu, then ringing, then a greeting, then a person, on one call without a hiccup between them.
Next
Hold is half of the pair. The other half is handing the call to someone else.