It could be due to the lack of VoLTE. But then again, if audio works with an external microphone, maybe it’s something else. I had a similar problem with my Sony at one time, but it corrected itself when I switched to a different mobile service provider.
See No audio from earspeaker during calls (since approx. e-1.8) and [UNOFFICIAL BUILD] Sony Xperia XZ1 Compact (lilac) e-1.3-q Post #212 - #222 by Taurus