Wi-Fi calling with AT&T seem only to kick in when there is no or low signal. To simulate this: On the phone settings -> Cellular turn on Wi-Fi calling. Then turn off Cellular data. Wait a little bit and the phone should switch to AT&T Wi-Fi calling. After that go to the Watch App -> Phone Now the Wi-Fi Calling slider is visible and can be turned on. Turn your Cellular data back on. Hope this will work for you.