Google does say that the "full Google Fi experience" might not be available to any ol' phone you bring over and is instead reserved for phones "designed for Fi", like the ones they sell. This sounds like a big improvement from the handful of Fi-compatible devices, but that additional device support is a little complicated.
With the advent of Google Fi, the company says would-be subscribers can get on board with "many Samsung, LG, Moto, and OnePlus devices", as well as iPhones (as part of a beta program). What makes Google Fi unique is that Google licenses service from multiple cellular companies and bundles them together-specifically, using Google Fi in the US means your device will combine the T-Mobile, Sprint, and US Cellular networks and will jump among them as needed.
However, there may be a catch: these devices might not give their owners the "full Google Fi experience", potentially meaning a few Fi-specific features, such as the aforementioned automated network switching (one of the service's main selling points), won't arrive for non-Fi-optimized phone owners.
With today's announcement, however, none of the aforementioned issues should crop up anymore.
IPhone users can sign up for Project Fi with the new Google Fi for iOS app, which is now in beta. But in general, iPhones will have access to calls, texts, and data powered by T-Mobile's network. New or existing customers buying their phones from Google Fi can get the value of their phone of choice through any mixture of gift cards for Airbnb, Hotels.com, Delta and Southwest Airlines. No matter what phone you use with Fi, you'll get great Fi features-like reliable coverage, easy group plans, and high speed worldwide data coverage for the same rates you pay at home. But you'll also be getting coverage that is inferior to what a true Fi device gets. However, the offer is available only today, November 28, 2018, and will end at 11:59 PM Pacific Time (in 15 hours from the time we're writing this article).
Thankfully, no matter what phone you own, you still get Fi's other perks. Among other things, that marks the first time you've officially been able to use an iPhone with Fi.