Open Source code could be non-shareable. For example, all those repos on the internet without explicit license are Open Source but proprietary, because you cannot do anything more than read the code (i.e. no use, modification, nor share freedoms allowed)
I prefer using Free Software. I’m a Debian user for a reason. I believe in freedom, thus I avoid proprietary software and “just Open Source” software all I can.
Regarding Fairphone Open OS, I think it is as free as it can get, due to the binary driver blobs, but far from 100% libre. Android is released under Apache license, and I think Fairphone licensed their modifications under such license (although it seems they forgot some modules…).
The only fully free mobile OS is Replicant, but it isn’t fully functional. Freeing drivers is a hard and thorought work and requires a vast amount of time.