In recent years, WiFi calling has become increasingly prevalent on various carriers and devices; the recently-announced Pixel and Pixel XL back up this fact. Google has confirmed that the Pixel smartphones will support WiFi calling for both T-Mobile and Verizon upon launch.
It's not difficult to see why Google chose these two carriers to get WiFi calling first. T-Mobile was the first American carrier to implement WiFi calling on a large variety of devices and worked with Google last year to get it onto the Nexus 5X, 6P, and 6. Meanwhile, Verizon is the only US carrier that is actually selling the Pixel devices, so it would be pretty laughable if they didn't work with VZW's WiFi calling.
According to the rep David spoke to, WiFi calling could expand to other networks in the future, but don't count on it, as we all know how that goes.