[Android Q&A, Help & Troubleshooting] Sass86oh:
Yea, here's everything you need to know about using Project Fi on unsupported devices. This is from my own personal experience and it took a while for me to learn everything because there are so many different posts on line concerning this topic and I've realized that most of it is inaccurate. I hope this helps you out!

Activate Project Fi account from an unsupported device (Successfully Accomplished)

**Although I accomplished this on a OP5T, theres no reason why the steps below shouldn't work on just about any other Android device, the Fi app is what activates everything as well as sets the proper settings for MMS and VVM. As long as the Fi app believes the device it's installed on is one of the officially supported Fi devices then it will proceed with the activation of a new sim card**

Project Fi is an amazing mnvo with one absolutely annoying bottleneck... Your service can only be activated from a handful of devices. You can sign up for service and you'll be sent a sim if thats the route you want to go and the account can only be activated for use once you've signed into your account (with new sim inserted) on one of those phones (5 total I believe)... Now once your sim is activated it can then be used just fine in any T-Mobile supported phone (with a loss of the ability to transition between multiple networks as supported by official devices) but that's the only downside. If you have great T-Mobile service in your area then you're fine and you could have cell service for like $27 a month....

I've been experimenting with activating service on a phone that's not officially supported and I am here to tell you that I've successfully managed to do so using a OnePlus 5T which is not an officially supported Fi device. The process was insanely simple and I've actually wondered from day one whether or not the process I accomplished this with would work, but I figured it was too simple for Google to not have considered and so I never attempted it. This was literally the final attempt I was going to make on this and it worked flawlessly.

STEP 1

Root your phone, go /system and edit the build.prop.... (/system means the "system" folder in the root partition, known as "/")

Change the following entries to what I have indicated, (you're only editing the words after the "=" sign)

Each entry below will say something different following the "=" sign depending on which phone you happen to be using... Everything to the left of the "=" sign should be universal across all Android devices...

So if you have a OnePlus 5 for example, then you will initially see "ro.product.model=OnePlus 5" and "ro.product.name=cheeseburger" you'll only change "OnePlus 5" and "cheeseburger" to match what I have written below....

***THESE OPTIONS ARE CASE SENSITIVE!! SO IF YOU WRITE "Taimen" INSTEAD OF "taimen" THEN IT WILL NOT WORK***

ro.product.model=Pixel 2 XL

ro.product.brand=Google

ro.product.name=taimen

ro.product.device=taimen

ro.product.manufacturer=Google

ro.build.product=taimen

ADD THE FOLLOWING ENTRY TO THE BOTTOM OF YOUR BUILD.PROP (the complete line, not just "true")

ro.opa.eligible_device=true

STEP 2

Save the new options added to build.prop and REBOOT YOUR PHONE BEFORE PROCEEDING! If you did everything correctly, then after a reboot when you go to "system" and then "about" you should see the device info reporting that your phone is a "Pixel 2 XL". This is how we are managing the activation, because the Project Fi app will check the device info when you open it the first time and based on the info it sees, it will either treat the phone as an official device, or one that's using a data only sim (which can not activate Project Fi account). Data only devices are restricted in that LTE speeds are reduced substantially, VVM does not work, it's possible to call and text with a few tweaks but MMS will not work outside of the Hangouts app. Official devices have no restrictions. As long as it believes the device is official, it will allow the activation to proceed regardless of the device you're using.

STEP 3

(For the next part:

DO NOT GRANT PERMISSIONS TO PROJECT FI APP UNTIL AFTER YOU'VE OPENED THE APP AND ACTIVATED THE SERVICE!! Upon completing this task you can then grant the necessary permissions to Project Fi app. If you have already granted permissions then clear app data for Project Fi app, uninstall, reinstall and follow my directions)

Download "Project Fi" app and insert the sim you ordered from Google. Open Fi app and choose the correct account (if theres more than one google account signed in on your device) and if you've done everything correctly then the service will activate. I've done this successfully on 3 different devices already.

Once you've completed this you can then revert all changes to the build.prop back to original settings and your account will be activated and the sim can be used in any GSM device compatible with T-Mobile's network from then on.

FYI:

If you want to be able to send mms messages from any text app besides Hangouts and have visual voicemail working as well then you should leave the build.prop the way it is after adding in the Pixel info. All services (except for network transitioning, you'll be limited to T-Mobiles service which is amazing in most areas anyway) will function properly when the device is seen as being a Pixel 2 XL. If you revert the build.prop back to stock following the initial activation of your sim card the you will lose visual vm, mms messaging (except if you don't mind using hangouts for text and mms), you'll have to add the correct apn info, and you'll only get LTE speeds of about 10 Mbps. Leave it the way you changed it to for activation and the phone will function exactly as it does on supported devices with the ability to mms from any text app, working visual vm and the APN will configure itself and your LTE speeds will improve substantially. I hit 60 Mbps consistently. There's an easy way to tell. If your carrier label states "Project Fi" then the build.prop is showing the phone as a device thats not officially supported and you'll have to deal with the extra issues I've stated above. If the carrier label reads "Fi Network" then you phone is seen as a supported device and everything will work perfectly with the exception of seamless network transitioning.

Hope this helps some people out!