For WiFi calling to activate, first of all, you need to have reliance carrier. Second, you need to have a really strong wifi signal. In my own testing it seemed to be a combination of stronger signal than -50dB for a period of time. After WiFi calling was active, it took below maybe -75dB for a period of time before it went back to VoLTE.
I can imagine it's more complicated than this, like the stronger/weaker the signal is, the shorter time it needs to go between WiFI/VoLTE.
