https://www.cnbc.com/2018/11/28/google-fi-simple-phone-service-expands-to-apple-iphone-more-androids.html

Google's super-simple and cheap cell phone service will now work with a broader selection of devices, including iPhones from arch-rival Apple.

Google Fi (formerly known as Project Fi) is a mobile virtual network operator (MVNO), which means that instead of just using one of the "big four" carriers, it automatically jumps between several cellular networks depending on which has better service. Fi hops between Sprint, T-Mobile, and US Cellular, and also favors Wi-Fi whenever possible, including for calls and texts.