As of now, the Nexus 6 is the only phone that officially supports Google's Project Fi, the tri-band voice and data service where you pay only for what you use. I expect that to change based on the Moto X Pure Edition smartphone.
My Moto X review unit arrived earlier this morning and once I got it up and running, I did what any normal person wouldn't do: I put my Project Fi SIM card in and downloaded the Fi app.
Sure enough, the phone connected to the Project Fi network as evidenced by the settings screen. I had actually paused my Fi service last month and the Fi app asked on the Moto X if I wanted to resume it.
This support is currently unofficial of course. Google hasn't announced Project Fi support for any handset other than the Nexus 6. So there are a few limitations at the moment.
I can't force the phone to use Sprint's network so I don't know for sure if it would switch in places where that carriers signal is stronger than T-Mobile's. And the phone says it only has HSPA+ data services from T-Mobile; I'm not getting full LTE service. That's not a major concern; a speed test showed downloads in the 16 Mbps range and relatively low latency.
I was able to test the voice calling transition from Wi-Fi to cellular.
I called my wife's phone over Wi-Fi and after chatting for 30 seconds, disabled the Wi-Fi radio.
The call continued on the Fi network without a hiccup. I also disabled the Wi-Fi radio to test a standard cellular call without any problems.
Considering the Moto X Pure Edition -- a U.S.-only phone for now -- supports all major carriers out the box, this doesn't surprise me. It's the perfect candidate to become another official Project Fi phone because it has all of the hardware radio support to work on the Fi network.
And frankly, if Google wants Project Fi to take off, it has to expand the range of supported handsets.
Sure, the pair of new Nexus phones we're expecting later this month are likely to work with Fi. But not everyone wants a Nexus. Google has to work with hardware partners to include the multi-network capabilities so that Fi can work on an array of handsets.
I'm betting the Moto X Pure edition becomes the first non-Nexus to work with Fi soon after the new Nexus phones launch.