I can't seem to get it to work...
Go to Settings -> Phone -> WiFi Calling. There should be a tab under WiFi Calling settings that says something along the lines of "Add other wifi devices". You need to enter an activation code from the device into your iPhone to get it to register.I'm trying with an iPad mini wifi only. How do I do it? I works when the iPhone is on, but not when it's off.
Something which should probably be made clear is that this feature depends on whether your specific carrier supports it. As of now it's only available in the US, though AT&T, Sprint, or T-Mobile.
Additionally, I think you need to go to Settings -> General -> About to get the latest carrier update from AT&T.
Go to Settings -> Phone -> WiFi Calling. There should be a tab under WiFi Calling settings that says something along the lines of "Add other wifi devices". You need to enter an activation code from the device into your iPhone to get it to register.