I was in the US with my wife for the last couple of weeks. While there, Wi-Fi calling wasn't working on my iPhone 13. I was of the impression that Vodafone geoblocked it while roaming. But I later noticed that my wife's Galaxy S20 had the symbol in the top status bar. Her phone was in flight mode but with Wi-Fi enabled. I tried making a call on her phone and it worked.
So is it phone-specific or something?