Mac - Echos on phone calls

The other party often hears a loud echo of their voice (not mine) when I use FaceTime on my Mac Mini for phone calls. I hear no echos. This once was only an occassional problem but recently it's happened most of the time.

I use an external mic, of course, since Mac Minis have no mic. To improve the sound I hear for all my audio, I use external speakers. None of those devices has moved since I installed them seven months ago. I use this setup for Zoom calls with no problem.

I've seen that many others have had this issue, but none of those conversations here lead to ananswer. I hope someone here can give me one!