MacBook Pro (Retina, Mid 2012) OS X 10.10 (build 14A389)
iPhone 5s iOS 8.0.2 (build 12A405)
Here is how I got it to work:
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) On your Mac go to Apple / System Preferences and double check that under "General" you have "Allow Handoff between this Mac and your iCloud devices" checked
Make phone call from you Mac. Start iMessage on your Mac and finish it on iPhone, start email on your iPhone finish it on your Mac etc.