They all brand it differently but Apple call it "Wi-Fi Calling on supported iCloud-connected devices" on the following webpage:
https://support.apple.com/en-us/HT204039.
It looks like AT&T, T-Mobile USA and Sprint (not Verizon, my mistake) all support it. No networks outside the USA support it, not sure why that is. The reason i'd like this feature is for situations where my iPhone battery has gone flat, my Apple Watch will still be able to make/receive standard phone calls using the same number, including with non-apple devices, assuming Wi-Fi is available.