WiFi calling is a carrier-provided feature which allows you to make phone calls and send text messages where WiFi is available but cellular coverage is inadequate. You can call and text (SMS/MMS) as you do on the ceullar network.
You need to make minor configuration adjustments on your cellular phone to activate WiFi calling. Once enabled, cellular phones automatically use WiFi calling when needed.
WiFi Calling Details by Carrier
WiFi Calling and Emergency Services
During setup, you’ll be prompted to include an address for 911 emergency call routing. The address for 911 emergency call routing helps first responders locate you at the address where you are most likely to use WiFi calling. When you place a 911 call, it uses the cellular network when possible. If no cellular network is available, device location data from nearby WiFi networks can be used to help locate you. If that data is not available, the address you provide during setup will be used.