Its a conflict between the bottom speaker and microphone. They could fix it with an update and they pretend they did, but it's not fixed for all type of phone call. I am on samsung s10+. I made multiple test. I DO NOT USE SIM CARD. I only make call via internet application voip call. On one of the app I have to turn the speaker ON because the sound will stop being recorded from the bottom speaker. On the s21 it has to be something similar, but i can't tell exactly. On the s10 its definitely the bottom microphone conflicting with the speaker and sending the voice back. I called samsung but the service is so bad they telling me to bring to phone to a samsung center to have the phone checked for hardware issue. The thing is on another phone call app they use both speaker all the time or it has different setting according to microphone and speaker usage.
For exemple I purchase a Omen Blast gaming headset from HP and the microphone have a conflict with the speaker. The microphone constantly interfere with whatever is coming out from the speakers.
Voip call application has echoing cancelation options, but it is not software. I can turn echo canceling ON and OFF and there is also a 2e degree voice canceling and a 3rd degree for extreme cases. NONE OF THESE OPTIONS FIXED THE ECHO.
AGAIN: for s10 its is a conflict of the BOTTOM speaker and microphone interfering together. It is 95% chance the same thing on S21.
For s21 turning wifi calling OFF may fix th3 issue. I guess it changes the way your phone call application uses the mic and speaker.
I hope an expert technician from samsung read this a bring a definitive fix with his team. This is a very sad issue for such good phones
