If it's anything like the Samsung Galaxy Light, it's virtually impossible. With that device, the only way I could figure out how to do it was to Factory Reset and NOT set up anything with WiFi Calling.
There's probably ways to do it though -- clearing data for certain apps, I just don't know off the top of my head and it was convenient for me to FR since I hadn't set much up yet.
Are you using a T-Mobile Lumia 810? From the resources i was looking at, Wi-Fi calling isn't available on the device.
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