11/24/2018 - this worked for an iPhone 7 running iOS 12.
Turn off WiFi.
Select Cellular and turn on Cellular Data.
Shut down and restart phone.
Turn on WiFi and be sure you’re connected to your router.
Cellular Data should still be turned on.
Select WiFi Calling.
Turn on WiFi Calling.
After several seconds you should get a screen asking for your emergency address.
Fill out and submit that screen.
Leave WiFi Calling turned on.
You can turn off Cellular Data.
Turn on Airplane Mode to prevent the phone from using normal cell service.
Be sure your WiFi is still on and you’re connected to your router.
(At the top left-hand corner of your screen you should see the airplane, "XM WI-FI," and the WiFi symbol.)
Make or receive a call.
Turn off Airplane Mode so your phone will work normally.
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