Quick update: I think I may have fixed this issue.
I removed the SIM and tray, rebooted, activated wifi, and made an XMPP call to my home phone. I toggled speaker on and off (not sure if it was necessary), and audio is now reaching both phones in both directions.
Powered down, put the SIM and tray back in, and call audio continues to work normally.
I always insert SIM cards very carefully, making sure they don’t shift, so I’m not sure what the problem might have been. Maybe it just needed a SIM re-seat.
Maybe useful info for support, @Manoj.