Les, it's the other way around: when you enable WiFi calling, the phone uses WiFi by default to make calls. If no WiFi is available (or no strong signal), then it will fall back to cellular reception, if available. The main reason (for the phone user) to use WiFi calling isn't necessarily for no cell reception but for poor cell reception. I have spotty T-Mobile coverage at home (usually OK, depends where in the house) and sometimes enable WiFi calling because I know I have strong WiFI at home.
So you can test WiFi calling right now: just enable WiFI, turn on WiFi calling...wait a minute or two (it can take some time for the phone to figure out it is able to make good-quality WiFi calls), then try it out. You'll see that WiFi calls show different icons and notifications. WiFI calling is free on post-paid Simple Choice plans but perhaps not with pre-paid if that's what you have right now.