Yeah, I tried this, didn't work for calling over wifi. HD voice is enabled, Use WiFi is enabled, GS7 is connected to WiFi. But when I try to make a call from this location that has no cell signal - I get mobile network not available error message, still using the cell radio, not using WiFi calling. Using Android 8.0, Samsung Experience 9.0 and Baseband version G930VVRUACSF2.
Note that my IPhone does work, so I think this is a Version specific problem because iPhones are naturally global devices where as Verizon Androids are not.