One of the requirements of making a call through Wi-Fi calling is that the iPhone must have a Wi-Fi connection. This Wi-Fi connection is required for carrier to make an authentication and then continue with making and receiving calls through that Wi-Fi connection, This means that if you do not have a Wi-Fi Network or your iPhone is not connected to a Wi-Fi Network then Wi-Fi Calling feature cannot be used.
The other question then pops up is whether one phone number can use another phone numbers Cellular connection to use for Wi-Fi calling. The answer is still a NO.
Can you use a HotSpot connection for Wi-Fi calling? Answer is YES, however, it cannot be from a HotSpot created by the same device that makes the Wi-Fi calling phone call. In other words, your iPhone would have to be connected to a HotSpot created by another separate device.
Axel F.