As I already received some tens of questions in my office about it, here is what I did when I came back to France with my two available phone lines (France Telecom, my regular phone, and Free, the internet one).

My view was that it was not acceptable to have two phones devices just because I got two phone numbers. For instance, if I am already busy on phone, there is no way to answer a second incoming call, and I want this second call to give the caller a busy signal.
Also, I wanted to be be able to choose the outgoing line depending on my own reasons (price, destination, etc...), but again with the same phone device.

One answer could be:
Just Google a little by yourself (this is what I did a few days after I understood the issue), or have a visit to National Communication web site, and search for their Line Selector.

I hope you this could help some freebox users :)