You tried to find the feature in the phone but it was nowhere to be found.
After many searches on Google,I found out how to enable this:
- Go to the phone app
- Click on the three little dots next to the search bar on the top left hand side of the screen to go to phone settings.
- Click on settings.
- Scroll down in the settings and click on wi-fi calling.
- Select "Wi-Fi Preferred".
This should activate wi-fi calling in your phone!
For other interesting settings you can visit this support page at T-Mobile: