This does not refer to FaceTime calls, but rather regular calls made over cellular to ordinary phone numbers. Ringtones come through the Mac the same as they do on the phone. The same is true for call waiting calls, which you can accept or reject on the fly.
To dial from the Mac, hover your cursor over a phone number wherever you see it (in Contacts, Calendar, Mail, Messages, or Spotlight), and then click the phone icon that appears adjacent to that number. You can also highlight a number you come across on the web in Safari to initiate a call. Or dial conference calls with a single click from the Calendar (passcode included).
The only requirements are that your iPhone and your Mac must be signed into the same Wi-Fi network and iCloud account. And because calls are routed through the iPhone, the phone has to be fairly close to but not necessarily in the same room as the Mac. Oh, and go into FaceTime preferences and make sure the Calls from iPhone check box is selected.