Did you know even if you are connected to Wifi on your iPhone, when you make a call it may still go through Celluar?
To ensure you are calling using Wifi, make sure you do this first.
Settings > Cellular > Wi-Fi Calling turn it on
Then do this:
The solution to make sure Wifi calling stays on:
1- In the Control Center, tap the Airplane Mode button.
2- Tap the Wi-Fi button to turn Wi-Fi on if you haven’t set Wi-Fi on previously in Airplane Mode.
3- Wait a moment, and the carrier’s name plus Wi-Fi should appear, even though you aren’t connected to the cellular network.
4- Turn off Airplane Mode. The Wi-Fi icon should remain in place.
Now when you call and are connected to Wifi, it will use Wifi to make the call...
Why use Wifi to make calls? Much better connection and bandwidth then celluar.
Also if you have a limited data plan, Wifi calling does not use data from your plan.
Keep in mind this is for calls, whether it be phone calls, Facetime, Email, surfing the web and or streaming videos, it will all be via Wifi...
Enjoy saving on your data plan.