My iPhone, my Mac mini, and FaceTime

The requirements for FaceTime audio and video calls on a Mac, Apple tells us, are to have “a built-in or connected microphone [or] for FaceTime video calls . . . a built-in or connected camera.” I have dutifully purchased, connected and successfully used both to make calls on my Mini, but why is it that I can’t just plug in my iPhone and use that? Or can I? The idea seems like such a simple and useful one that I keep thinking I must have missed something somewhere, sometime . . .

Thanks for your patience.