What could cause outgoing phonecall audio to fail?

My Moto G6 Play runs Android 9. Some time in the last year or so (I don't make a lot of phone calls) people started telling me they couldn't hear me, or only very quietly, when I called them. It's not the microphone because I can use that to record audio on an audio-recording app. It's not the sim because the I can use that in another device without a problem. It's not the default Google phonecall app because the same problem occurs with e.g. Viber. Location might be a factor because I've noticed the problem at home (where I make almost all of my rare phonecalls) but people seem to have been able to hear me during some brief outdoor phone calls. On the other hand perhaps I was speaking louder. Any ideas? ---------- **ETA:** It's not a "two microphone" issue because turning on speaker-phone during a call doesn't make a difference. I can't find any noise-cancelling setting that could be affecting it.