Asked if I could utilize other phones which seem to work to some degree.
Don't get me wrong, my Nexus 6 is a great phone.
Changing phones is just one of my [least] bad habits.
While the answer was unambiguous, wish I knew what functionality was missing.
The reports I've seen from folks who claim that a Fi SIM "works" in other phones generally say that they CAN connect to T-Mobile, but the phone never auto-switches between T-Mobile and Sprint and they can't do WiFi calling. Which isn't surprising - Google has complete control of the Fi software, so they can guarantee that even if the SIM says you're allowed to connect to T-Mobile or Sprint, and even if the phone has a radio that supports any carrier, the Fi software will recognize that the phone is not a Nexus 6, 6P or 5X so the software will refuse to perform all the Fi tricks.
Just as any carrier reserves the right to qualify/certify phones for their network, Google reserves the right to do the same for their "network" by controlling which phones the software functions on.
I wish it were otherwise, but until Google changes Fi from an invite-only "Early Access" program we just won't know if they plan to open Fi up to other phones, and which phones that will be. I'm sticking with Fi for now because I can live with the mostly-WiFi very low cost model, but I really hate not having the usability features of the Moto X and the option to pick another phone if my preferences change - so Fi is still on probation as far as I'm concerned. If Fi turns into another of Google's never-ending beta programs then I may boot them anyway.
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Alex