I have read through Make a FaceTime call using AppleScript, but the provided solution is for starting a video call. I am wondering if that solution can be adapted to make a FaceTime Audio call (and by extension, maybe even a call via iPhone utilizing Continuity).
I found it!
I tried many different combinations, such as
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: