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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.
 
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.
We’ve used WiFi calling with no issues in our home for over 7 years. Just within the last 8 months or so, a new tower was activated for AT&T less than a quarter mile from our home, with a very strong cell signal and actually faster download speeds than our Fios internet service. However, our devices remain 100% of the time on WiFi. The AT&T cell network has no impact on that function. The only time ours reverts to the cell network is when we’re a distance away from our home. Until 8 months ago, we had very poor service in our community on AT&T; now it’s terrific, but we still always use WiFi at home.
 
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