You have two options when you sign up for Project Fi. You can buy a SIM card with a new Nexus 6. Or you can just buy the SIM card and stick it in your own Nexus 6. Want to use another phone? You can, as long as you tether it to a Nexus 6. A Project Fi SIM has to go into a Nexus 6.
For the time being, that eliminates a sizable chunk of interested consumers.
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