How do I completely mute the sound in phone calls? I don't mean I want to mute the ringtone, and I don't mean I want to mute my own microphone. I want to mute the sound of the other person on the call.
When I use the volume control during a call, I can put the slider all the way down to zero but the call is still audible - either the slider only acts on the ringtone, or the slider is capped at a minimum (non-zero) volume level. How do I make the call completely silent?
(For context: I make calls using a relay service, where I type, and a remote assistant speaks and types the response back to me. This service involves making the call on my phone, so I still get the sound of the people talking - I want to silence this.)
Indeed there is a minimum output volume for the audio coming from the call.
There might be a hidden settings, although it might be difficult to find or to apply to some phone models or Android verions.
There is also an option that is probably applicable to all phones: you can plug any headphones using the audio jack (assuming your phone has an audio jack, which perhaps all Android phones still have).  This redirects the audio to the headphones, and if you don't wear the headphones you will not hear it.  Which is what you were asking for, isn't it?
Notice: if you plug a headset (headphones with included microphone), your phone will most certainly use the headset's microphone, which may or may not be what you want.  If you plug headphones (without microphone) your phone will continue to use its internal microphone.