Using T-Mobile with my factory unlocked A15, I was able to use WiFi calling. However when I switched plans to AT&T, the option for WiFi calling straight up disappeared. I checked if VoLTE was enabled, which it was. Also IMS registration service is ON. I'm not entirely sure what caused this. My plan definitely supports WiFi calling but I'm planning on calling AT&T if this post doesn't get any attention.
It is possible that AT&T does not support wi-fi calling on that model. You would have to speak to AT&T tech support, since wi-fi calling is a carrier issue. You may have to factory reset the phone to clear out any T-Mobile stuff. Make sure you use Smart Switch to make a complete backup to a flash drive or computer before resetting.
