Why can't I hear any phone calls?

My phone is a Nexus 6 running Android 7.1.1 with all available updates installed. Recently, I cannot hear *any* audio during phone calls, not from the earpiece and not from the speaker. I tested them with other audio sources besides phone (like podcast) and they work fine, so it isn't a hardware problem. I've reset the phone (including resetting bootloader) a number of times. I reset all app preferences, and then reset the phone a number of times and it still isn't working. I installed "SoundAbout" to try to fix the problem, and tried many combinations of settings, and it still doesn't work.

I've seen this problem reported before, and I've tried everything recommended and it still doesn't work. It's just so weird!

Any ideas?