It's entirely up to the carrier when your phone uses Wi-Fi calling.
For example, in my experience AT&T bases it on signal strength and whether the nearby tower is heavily loaded. It seems that in good signal conditions, AT&T would rather carry calls on its own network. This is understandable because AT&T has no control over whatever Wi-Fi your phone happens to be on, so there's a good bet that people with a strong cellular signal will have a better call experience on cellular.
In my office at work, which only has 1-2 bars of slow and unreliable cellular, my phone stays on Wi-Fi calling 100% of the time. At home where I have a huge cellular tower only a couple blocks away with full bars all the time and 600mbps speedtests, Wi-Fi calling is off a good 95% of the time. There are exceptions, and I suspect that it's when the tower is heavily loaded.
For example, in my experience AT&T bases it on signal strength and whether the nearby tower is heavily loaded. It seems that in good signal conditions, AT&T would rather carry calls on its own network. This is understandable because AT&T has no control over whatever Wi-Fi your phone happens to be on, so there's a good bet that people with a strong cellular signal will have a better call experience on cellular.
In my office at work, which only has 1-2 bars of slow and unreliable cellular, my phone stays on Wi-Fi calling 100% of the time. At home where I have a huge cellular tower only a couple blocks away with full bars all the time and 600mbps speedtests, Wi-Fi calling is off a good 95% of the time. There are exceptions, and I suspect that it's when the tower is heavily loaded.