Your iPhone and Mac are signed in with the same Apple ID and iCloud and FaceTime are turned on. See Connect your Mac to the internet using Wi-Fi. Your iPhone and Mac are on the same Wi-Fi network and connected to the internet.
See the Apple Support article Update your iPhone, iPad, or iPod touch. Your iPhone has iOS 8 or later installed. To make and receive phone calls on your Mac (OS X 10.9 or later required), make sure of the following: What you need to make and receive phone calls on your Mac
