Project Fi automatically switches between networks depending on signal strength and speed. Phone calls, if placed over a Wi-Fi connection, will seamlessly transition to a cellular network if Wi-Fi coverage is lost. Project Fi users can use Google Hangouts on any phone, tablet or computer to call and text.
Only a few phones have he capability to alternate between networks (Nexus 6 and Pixel phone product lines), however, the compatibility list if growing and you will soon see your favorite phone capable of using Project-FI's full benefits.
First launched in 2015, Google's cellular service started to only be for people who knew it existed, and even though you knew about it and wanted to join, there was a waitlist you'd have to get into before you are sent an invitation code to ditch your old carrier and start using Project-Fi.