@pluralism here is something that might be worth trying, see this comment in another thread: L5 call audio quality - #86 by dos
sudo rm -rf /var/lib/alsa/*.state ~/.config/pulse
sudo reboot
Several people in that thread answered that the above gave an improvement for them. I think it did help for me also at some point. (I suspect there are bugs causing those files to end up in a bad state even if the user did not explicitly mess with any settings.)
About the “external mic” thing, I don’t know about that. I usually don’t use antying like that, I just use the phone itself with nothing external attached. I don’t know if call audio would work better or worse if some kind of external mic was used.