Google Project Fi is not accessible with all the devices as it requires high-end smartphones which have support for it.As of now only Pixel, Pixel XL, Nexus 6P, Nexus 5X and Nexus 6 are compatible.
The cheapest device which supports Google Project Fi is Nexus 5X which is bit old and outdated for now.But we can expect now a mid-tier device from Google which supports Project Fi as Google teases about it on Twitter yesterday.
We hear you loud and clear. Keep an eye out for a new Fi-compatible device at a mid-tier price from one of our partners later this year. https://t.co/74U3bq16pa
As of now, we are not sure whether it will be called Pixel or not.As per the tweet, they have mentioned with partners which mean it will be the first Non-Google smartphone which will be compatible with Project Fi, which helps Google to increase the range of smartphone which is compatible with Google Project Fi.
We have to wait for more details about the mid- range smartphone and which manufacturer is launching such a device.