https://www.theverge.com/2018/11/28/18116333/google-project-fi-apple-iphone

Google announced today that its Project Fi wireless network, now rebranded to Google Fi, will be available for iPhones and the majority of Android devices. That’s supposed to open up a lot more phones to Google Fi’s unique features, such as flat rate data billing; unlimited international roaming; and the ability to switch between multiple networks, like T-Mobile, Sprint, and US Cellular, for better service. However, if you’re planning to use your iPhone on Fi, there are quite a few caveats to be aware of.

Though Fi SIMs have unofficially worked on non-Fi phones (including iPhones) before with some limited functionality, Google is now supporting iPhones directly. You can go through the Google Fi site here and check if your phone is compatible, but to save you some time, you should know that iPhones won’t be able to make calls or text over Wi-Fi, use visual voicemail, or be used as data hotspots outside the US at all. They also won’t be able to take advantage of Fi’s unique network-switching features. Google Fi flat-out won’t work on the iPhone 5, 5C, or older.