Hello Neil22R,
Thanks for the response, but the problem really isn't an inability to make or receive Facetime calls, so the document is not really helpful to me. In fact, the audio on every single call starts out fine, but within a minute or two (sometimes a little longer) the person at the other end gets dropouts on the audio from me, around a second long or even several seconds, often several of them in a row, so that they can't even understand what I'm saying.
I engage in several Facetime calls with that same user every day (and have done for the past couple of years), and we never had this problem until I upgraded from my iPhone 6s to an iPhone 13. The other user in our house who does regular FT calls with that same person is still using a 6s, and has no problems. The only thing that has changed is my phone. It doesn't matter whether I am using WiFi (which is over fiber and has plenty of bandwidth), LTE or 5G.
When I look around the internet, I see that I am not the only person with this problem. I didn't spend a bunch of money to have a worse phone than my old one. If I can't resolve this issue soon, I will be going back to the dealer and cancelling my upgrade, and going back to my 6s.