In addition to these three, with their individual volume sliders, three more volume sliders appear, only when a call is in progress through Calls, just below VolumeLevels>SystemSounds on the Settings>Sound page. There are icons at the left side of the page beside these three sliders. The first icon is something I can not decipher. The other two show a stylised picture of a smartphone, accompanied by the word “Calls”.
It would appear that the two sliders named “Calls” might reasonably be thought to control microphone and speaker volume for the duration of the call, overriding any settings lower down the page. [EDIT 11 March 2023: Yes, I have tested this and that is what those temporary additional sliders do.] Does anybody know whether this is in fact the case? If not, what do they represent?
In any case, with both of those temporarily present sliders set to their maximum, there is still no audio going out to the VoIP server during a VoIP call through the Calls app.