My voice echos when I'm on a call. Also happens while using my loud speaker or through car stereo system. I can hear myself echo, but not the person I am calling.
I get this often as well, mainly when I'm using the speakerphone. It's been happening on my s21 ultra but not on my previous s10 plus. - Let's hope someone comes up with the reason why it happens and a solution to this as its REALLY annoying at times!