It is a carrier option that's only available with phones sold by each carrier unless you flash your device with carrier-locked firmware. It's not available on unlocked devices.
actually, I recently bought an S21+ and I can confirm that HD calling worked and video calling through Verizon was an option although I never tried it. their first OTA update kicked the settings off and I have been fighting every tech support known to man trying to fix it. no one knows anything however so hopefully samsung pushes an update that fixes it
