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.
WiFi Calling is Supported by Xiaomi 11T Pro
No, officially, it doesn’t support the WiFi Calling feature, but you could still use it by rooting your device, but it is harmful.