Support for VoWiFi is a bit weird. Some operators don't allow it outside of your region (for example, the EU), some others will allow it and even bill you as if it were a local call made from your country.
Also, although it's in theory made only for working through WiFi, seems like if you have a second SIM or eSIM with a data plan (for example, a travel SIM local for the country you're on vacations) and some combination of phone and/or operators, it can use the second SIM as the data provider for WiFi Calling.
I was using a Samsung S5 running LineageOS for a while with Google voice for calls, but the no 911 thing is one of the main reasons I finally moved to a different phone. The S5 supports VoLTE but only with stock firmware.