I had a similar issue with my iPhone 13 Pro. Whenever I use speaker phone, which is most of the time, the microphone will drop out. People would keep asking me to repeat what I was saying or tell me the line was bad. I took up the issue with Vodafone, who advised me to speak to Apple Support.
A friend suggested I took my sim out and put it in another phone. I still had my XR hanging about, so I put my sim in it. Hey presto my calls were, so this wasn't a carrier issue... This was clearly a problem with the iPhone 13.
I contacted Apple Support, after an hour going through settings and trouble shooting with them, they requested I send the iPhone back so it can be looked at. After about 10 days I was informed a replacement phone was being mailed to me.
Guess what? The new phone 13 is doing the same thing! Really annoyed as this is a premium phone £1,250! I contacted Support again last week and they have requested I mail in my phone so it can be examined. I'm reluctant to do this until after Christmas as I don't want to be without my Pro.
Apple need to sort this issue and I don't think it's software related.