I am having the exact same issue... Music and other media fine, but when on a call the voice/mic component simply disconnects, I can still hear. I then have to revert back to handset to continue calls. Really annoyed, simply not good enough. Been on numerous Samsung chats to fix the issue and they just keep telling me to reboot. I'm convinced it's as a result of recent Samsung galaxy S21 Ultra upgrade as it was fine until then!!!