https://phys.org/news/2018-11-google-fi-mobile-iphone-android.html

But at least one major factor has held Fi back: The service worked only on a few phones, Google's own Nexus devices back then and eventually Pixel devices and select Motorola and LG handsets.

On Wednesday, Google Fi takes a bold step forward, by adding compatibility with most Android devices, including models from Samsung and One Plus—and yes, for the first time, the iPhone.