Whilst in a call use the volume rocker button to see if that increases volume.
If not and your ports / mics are free of lint etc then a Samsung Service Centre can run a diagnostic test on the phone.
Daily Driver > Samsung Galaxy s²⁵ Ultra 512Gb ~ Titanium Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "
