I just called the Skype echo test service to have a listen to what my mic actually outputs. It's pretty bad! The sound is very muffled and almost inaudible unless I cup my hand over the mic and speak clearly. Removing the case doesn't seem to make any difference.
Sony Xperia Z5 Compact, if anybody's wondering which phone not to buy for regular phone calls :) I don't really mind too much as I only make phone calls once in a blue moon, but it would be nice to be able to use voice assistant more.
It seems that with a bit of bad luck, the mic can get clogged up if a drop of water ends up in it.
Try to hold the phone in running water so the mic gets wet, and then carefully blow out the remaining drop of water.