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No you can't use your own router instead of the freebox.
No you can't use your own router instead of the freebox.
Well you can, but you'll have degraded performance and not access to tv/phone services.
Well, you can, but you lose a whole bunch of important functionality, like being able to watch TV over IPTV (source - I work on SFR's STB, and we watch the competition closely :D)