This happens for both incoming and outgoing calls, with or without earphones, and regardless of whether I turn speaker on or off. The only way I can hear and be heard during calls is to enable WiFi calling, set it to WiFi preferred and call over WiFi while connected to a WiFi network. I am WELL within coverage area since my 4g LTE speeds are 25.91 Mbps DL 15.97 Mbps UL (signal strength -76 dBm 64 asu) so I shouldn't have to call over WiFi to make calls. Choosing "cellular network preferred" and completely disabling WiFi calling both result in me not being able to hear or be heard during calls.
I doubt this is an audio hardware issue (or anything to do with toggling volumes) since, as I've mentioned, calls work perfectly the moment I enable calling over WiFi. (I test it by calling voicemail-- with WiFi calling, I can hear the greeting, and without it I can't. ) Rebooting does not resolve the problem-- only calling over WiFi resolves the problem, which means I can't make or receive calls whenever I'm outside WiFi coverage.
One user suggested that it might be a problem with handset vs speaker, but I have Google Voice and have used it to test both handset and speaker... speakers and both work fine. (To listen to VM on Voice you can toggle between handset and speaker).
No water damage. No recent physical blows. Doesn't even have a scratch. Never rooted, never flashed, never fiddled with except for OTA updates. I don't think I installed any new programs between the time when my calling was fine and when it started going wrong.