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 here on my 5s. No WiFi Calling option under Settings> Phone.
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.
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!