Thank You Nelson Eduardo Pereira!
Now my MacBook Air and iPhone 4s connected like never before!😎
Yes, outgoing phone calls in Yosemite is working as soon as time is set automatically on all devices!
So, here are the correct steps to activate this cool feature (thanks to Chatlanian):
1) Make sure that your iPhone and Mac are on the same subnet (could be on WiFi and ethernet as long as they are on the same subnet)
2) On the iPhone go to Settings / iCloud and make sure you are signed in with same Apple ID to iCloud as on your Mac
3) On your Mac go to Apple / System Preferences / iCloud and make sure you are signed in with the same Apple ID as on iPhone
4) On the iPhone go to Settings / FaceTime and enable "iPhone Cellular Calls". On that same screen make sure you are using same Apple ID as in step 2 and 3
5) Restart your iPhone
6) Set date and time automatically on all devicesâš On you Mac with OS X 10.10 go to System Preferences -> Set date and time automatically; on your iOS device 8.1 go to Settings -> General -> Date and Time
P.S. forget about "Allow Handoff" as said Lanny "Your item #6 is unrelated to the issue. Hand Off and iPhone Cellular calling are two different a separate features." He is right.
and now, finally, all is working as Apple said http://www.apple.com/osx/continuity/:
Now you can take iPhone calls right on your Mac. You can answer a call or dial a new one while your iPhone may be charging in another room or even buried in your backpack. Just point, click, and say hello.
Making a phone call from your Mac is just as easy as you’d expect.
That's easy, isn't it?🙂