I went here to find the setting.
Place a Wi-Fi call from your iPhone
Turn on Wi-Fi calling in Settings > Phone > Wi-Fi Calling. You might need to enter or confirm your address for emergency services.*
Same issue for me. The wifi calling option just isn't there. I looked on my wife's iPhone 6 and found it on hers. Weird thing is when I tried to turn it on it said that she isn't authorized and has to call AT&T.Same issue here on my 5s. No WiFi Calling option under Settings> Phone.
Hmmm... I could have sworn I got a new SIM card with my 5S. I moved to it from a 4S which had the old style Micro SIM. But what you are saying does make sense for the authorization error my wife is seeing. Her SIM card in her 6 came from her 5. So one phone issue down, but I can't figure out why the option is not appearing for my phone at all.For those with issues, the SIM card that is in your phone had to come with a iPhone 6 or 6s because of voice over LTE. My daughter iPhone 6 isn't working because I put her old iPhone 5 SIM card in it when she upgraded. So off to AT&T to get her a new SIM card.
Thanks Jon. Yeah I don't know why I didn't think of doing that. It works for me too. Call quality sounded close to the LTE quality but there was a slight difference.Just turned off cellular completely and was able to still make and receive calls with "Wi-Fi" showing at the top of my screen, in between the "AT&T" and WiFi symbol. Woohoo!