Project Fi is the Google Carrier Service! For now you can only use it with the Nexus 6 that is Google has said that you must use a Nexus 6, but one savy XDA member has discovered a way you can use Project Fi on the Nexus 5 and possibly other devices. Getting Project Fi to work on a device other than the Nexus 6 is actually a pretty simple process. You will need to first activate your Project Fi sim on a Nexus 6. This could be tricky if you don't own the device. You may need to get creative. See if a friend would let you use theirs. Once the sim card is activated you will need to use a nanosim to microsim adapter to insert the sim into your Nexus 5. You can data wipe your phone, but you don't have to. Then install and open the Project Fi app. Your Nexus 5 should now be activated with cell service.