The voice-over-IP (VoIP) technology is mainly known as WiFi Calling. It is an extra service on top of your typical voice and internet plans, does not require a cellular signal from all carriers, and has no roaming fees. Does the Xiaomi 11T Pro offer WiFi Calling? Take a look at it from here.
No, officially, it doesn’t support the WiFi Calling feature, but you could still use it by rooting your device, but it is harmful.