If you are on the Project Fi network, Google can use open wifi networks for making calls. You'd need to leave wifi on to take advantage of this.
This is taken care of by google, so I never connect to a network/sign-in, etc.
This is taken care of by google, so I never connect to a network/sign-in, etc.
Edit: To be clear I'm not questioning your statement. Just curious to hear where Fi takes advantage of this.
There are several other places where it doesn't use the network unless I explicitly select it though (eg most Philz coffee locations, my work, etc)