Thanks, Nubz, but I'm afraid I can't get it working even by following all the instructions above.
1. I am signed into my one iCloud account on my Mac, iPhone and iPad. I can tell because when I check the accounts on each, they all list the same iCloud Apple ID email address, and they all offer for me to "Sign Out."
2. My Mac is running Yosemite 10.10.1, and my iPhone 6 is less than a month old, so I know it's on the latest system
3. All devices are connected my home Wi-Fi network
4. The Face Time account for all my devices are showing that I'm signed in using the same Apple ID
5. Wi-Fi calling does not appear as an option in Settings>Phone on my new iPhone 6
When looking at my iCal, all my upcoming appts. show the clients name, address and finally, phone number. None of the phone numbers look like a clickable link, they are the same font/color as all the rest of the info for each client. Clicking them does nothing.
The only way I was able to successfully make a call was to double-click the entire iCal appt. entry, which brings up the edit window, then right-click on the phone number in that window, which gives me a drop-down menu that includes "Call 698-XXXX" Using iPhone" and scroll/select that option. That worked and the call went through successfully. However, I'm assuming that's still wrong, as neither you nor the instructions on Apple's site say anything about needing to do those extra steps - opening the appt. editor, right-clicking the #, etc.
Additionally, there is some confusion between the instructions on the page you sent, under Turn Off iPhone Cellular calls. The page says, "Look below for instructions on how to turn off iPhone cellular calls." but then in those lower instructions it says, "On your Mac, open the FaceTime app and go to FaceTime > Preferences. Click Settings and deselect the iPhone Cellular Calls option." Seems like deselecting would negate using "your iPhone cellular connection to make and receive calls when your iPhone is nearby and on WiFi," not enable it