For the last couple weeks, people had been complaining about being unable to understand what I was saying on my PinePhone during phone calls (no matter who initiated the call). I could usually hear the other end very well, but not the other way round. (As I was told later, the problem was that the voice had some bizarre, hard-to-describe echo-type effect that made it pretty much unintelligible, no matter what microphone volume I set in the Plasma Mobile settings GUI.)
So, after a painful experience today where I was trying to call several people and none understood more than a tiny fraction of what I said, I had a debugging session in the evening with my mother, involving her calling me while we were both at home, and me playing around with alsamixer settings (from my desktop over SSH to the PinePhone, because the alsamixer TUI is much easier to operate that way).
Thankfully, I found the culprit pretty quickly (it was actually the first knob I tried): The Mic 1 Boost setting. For some reason, this was set to the maximum 100. I found that when I toned it down to some arbitrary value (it ended up at 45), the other end could understand me much better. Then I tried toning it down all the way to 0 and that made it even better.
This leads to the “public service announcement”: If you have an original PinePhone, you will want to set Mic 1 Boost to 0. Having it set to 100 makes for horrible sound for the other end of phone calls. For me, the setting was remembered once set, I did not have to explicitly save it somehow.
To locate the setting, either run alsamixer and use the F6 key and the resulting popup to switch from the virtual PulseAudio device to the real hardware where the setting is, or (especially if you are on the device and do not have a physical keyboard with an F6 key plugged in) run alsamixer -D hw:0 to get to the hardware device directly. Then use the cursor key to the right until the cursor is on Mic 1 Boost and the cursor key down to set its volume to 0. (Cursor keys can also be found in the special key row above the virtual keyboard in QMLKonsole.)