Ubuntu is an OS.
FreeNAS is NOT an OS, it's something that SITS ON TOP of a Linux OS, albeit a slim-down version.
I don't think FreeNAS can triggers backups but Ubuntu might. You can have the FILE SERVER to trigger backups for all of your client boxes, or you can have the individual clients to trigger the backups, is all up to you. I do not have the individual cases nor step-by-step procedure.