Have a built-in or connected microphone; for FaceTime video calls, you also need a built-in or connected camera
See Update your iPhone, iPad, or iPod touch.
Your iPhone and Mac are on the same Wi-Fi network and connected to the internet. See Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch.
Your iPhone and Mac have Wi-Fi Calling turned on. Wi-Fi Calling isn’t available on some Mac models. See TTY call.
See Choose a camera or microphone for FaceTime calls on Mac