This is happening on my sons Note 9 too.
After running diagnostics on the phone which shows the mic to be good I'm thinking it's software based.
What he does at the moment is accept the call then switches to loudspeaker and then switches back to the mic by turning off loudspeaker.
Give it a try @MathewDN
If you can I'd also advise to send this as feedback via your Samsung Members App.
Daily Driver > Samsung Galaxy s²³ Ultra 512Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "