I have a dual SIM Galaxy S7. I did some experimenting last night. With the latest S7 CSC update from 2degrees, Wi-Fi calling is possible. So I've flashed the NZC CSC onto my dual SIM (parallel import) phone. A reboot enabled the menu item in settings, and so have managed to get the WiFi calling symbol in the status bar, but interestingly, only when 2degrees is enabled as data provider.
The other thing of note, I've flashed the latest base ERF4 (newer than what is available OTA or via ODIN) and cell standby battery drain has dropped way down the list.
So all you on traditional update channels can only hope another update with change of base (pda) is forthcoming.