VoWiFi (Voice over WiFi) enables voice calls over your wifi network. This is also called Wi-Fi calling.
Here is how to enable it on the Xiaomi Mi 9T:
Open the dialer app and dial this number to enable VoWiFi feature: *#*#869434#*#*
Go to Settings -> SIM cards -> T-Mobile (or your provider). You should see a Make calls using Wi-Fi option here. Enable it if it is not already enabled.