Try giving this troubleshooting link giving additional information on microphone issues on your device: http://www.samsung.com/us/support/troubleshooting/TSG01001249/
What messenger app are you using? I would also try uninstalling and reinstalling the messenger app.
i have returned 3 different units to Best buy coz of the same issue. i had callers not being able to hear or hear their own voice when on spearkerphone while using Zoom Webex or Microsoft teams calls. The issue is the way the mic and the spearkerphone placement is right next to each other. I went back to my note 10 plus and there is no such issue coz they are on either side of the USB port. I suspect that also there is some optimization issues with the mics in general. I was very disappointed with my experience with the Note 20 ultra really wanted to keep the phone but even after trying 3 different units i had the same issue. Cannot take a chance with a $1200 device where the basic phone function doesnt work.
you can upload your problem on Samsung Members app.
upload your issue after you make a call with 3rd party app the the issue is saved in the log for R&D can review it.
I'm having the same issue , did they find a fix? Phone was like that since I bought it brand new, my phone works fine its my wife's that doing this, I believe there is a warranty for it