Making a FaceTime Audio call using AppleScript
I found it!

I tried many different combinations, such as facetimeaudio://[phonenumber] and facetime://audio/[phonenumber], but nothing worked. I was starting to think that there wasn't a way to do this.

However, today, I decided to take another crack at it with some different combos, and this one works: facetime-audio://[phonenumber].