Fi was limited to newer Pixel handsets made by Google and a few Android-powered smartphones made by other companies because devices need to be able to hop between carriers whose infrastructures are used to provide service on the "virtual" network.
The service, in essence, would unify all telecom operators, providing users 4G LTE speeds along with 24*7 human-to-human support.
The service besides ensuring smooth transitioning among multiple networks also securely connects the users to the available public Wi-Fi networks. A merger between Sprint and T-Mobile would essentially make Google's mobile service a relatively straightforward MVNO of T-Mobile, since U.S. Cellular's network only covers a portion of the country.
To get people on board, Google is offering either a travel gift card equal to the price of a new phone, or a $200 bill credit for folks who bring their existing phones, but both deals are only available today. These plans vary with group sizes and are charged accordingly. Google, however, charges users as they consume data. Phones that haven't been optimized for Fi won't auto-switch to Google's Wi-Fi, the same way they won't auto-switch through the three carriers. In the rare instance that someone uses more than 15GB in a single billing period, remaining cellular data may be throttled but is still free. First we need to talk about how Fi normally works. If you have a Pixel, though, or any of the other devices available through Fi's site, the cellular network switching feature is available (and extremely useful), and benefits like cheap roaming are also a major plus.
Google has published an amended list of supported devices, adding "Compatible with Fi" phones to its previous handful of "Designed for Fi" models (which were various Pixels, LG, Modo, Android One and Nexus units).
What's missing when you use an iPhone or non-Fi-certified handset, of course, is Fi's unique network switching. Other times, Google Fi will work in a way that is similar to the mobile data.
Google Fi is described as "a bill that's fair and flexible" on its official website, and that's true if you compare it to other USA carriers with purposely confusing plans and a long list of "incentives" with fine print. If you don't have those phones, the Fi will use one carrier: T-Mobile.