Note: To hand off a FaceTime call or use your iPhone as a webcam for your Mac, you must have Continuity Camera turned on in AirPlay & Handoff settings on your iPad or iPhone, and you must be signed into your iPhone, iPad, and your iMac with the same in the menu bar, click the menu next to Reactions, then choose a reaction. You can also share a reaction with just a hand gesture. Make sure the icon next to Reactions is green, and see Apple ID account, and that both have the feature turned on. (On your Mac, open FaceTime, choose FaceTime > Settings, then select “Calls from iPhone.”)
Note: Your iMac and iPhone must be connected to the internet and the same Wi-Fi network in order to make or receive calls on your iMac.
Learn more. See the FaceTime User Guide.