Project Fi is Google's long-rumored program to provide fast and easy cellular service directly to phones. It works by combining access to both T-Mobile and Sprint, with a SIM card that can actively switch between the two without any user intervention.
Project Fi also leans on Wifi, and lets you both call and text when connected to a Wifi network. Special software on the phone also automatically connects the phone to open Wifi networks to help limit your mobile data usage, and when it does so also routes your traffic through a Google VPN for safety and connection stability.
Prices start at $20 per month for the "Fi Basics," which is unlimited talk, text and keeping your account active. After that it's $10 per gigabyte of data you use in the U.S. and abroad, with international speeds capped at 256kbps.
Project Fi is a prepaid phone service, so you pay at the start of each billing cycle based on how much data you intend to use, aka $40 for 4GB, and are rebated on the next month if you use less than what you paid for. On the flip side, if you use more than what you paid for you'll simply pay the difference on your next bill at the same standard rate of $10 per gigabyte.
Beyond the simple pricing model and unique networking features, Project Fi also offers many of the core features that were available from Google Voice. That means you can forward calls to your Project Fi number on to other phones, as well as make calls and texts from other devices using the Hangouts app.
At launch Project Fi requires an invite, but Google intends to open up the service to everyone eventually.