The threshold is around -100dbm if you are in field test mode, which usually translates to 2-3 bars. Anything below there, and wifi calling usually switches on I'm pretty sure. It came on automatically for me and I have -109dbm (2, sometimes 3 bars where I'm sitting). Wifi calling can be forced if you turn on airplane mode, then turn wifi back on afterward. It takes some time for wifi calling to turn on sometimes, it took like 30 seconds to a minute to come on once.
It will say "VZW Wi-Fi" at the top when you are on Wi-Fi calling.
Screenshots attached for it, showing "bar" reading, field test reading, and it being enabled while on airplane mode. Not sure why I'm still on wifi calling when I'm at 3 bars, maybe once you go on wifi calling, that's the priority. I forced wifi calling with airplane mode earlier so we will see if it auto enables tomorrow when I come home again, won't mess with airplane mode to force it; just to see what happens.
It's also weird, I have such "bad" signal but I never have less than 70ms response time, or ~15mbps down and 5 up. Usually it's 20+. In the bathroom though with the worse signal (can drop down to -120) it still holds on at ~6mbps down and about 1 up. Perfectly usable for web browsing still. My house is covered by band 13 LTE, the far reaching, good in buildings type of LTE since I'm in a rural area, less than 1000 in my town lol. Pretty much the same as T-Mobile's band 12 LTE, just has a bit more capacity/a little speedier.
