In fact I think that not everyone can bypass the Orange line rental, it depends where you live. You can only do this if your geographical location is "dégroupé" (I think the English word is "unbundled" but I could be wrong). As I understand it, this is a purely commercial arrangement between Orange and the other providers whereby Orange allows them to take over an agreed number of lines, ie (I imagine) eg Free pays Orange a sum for n number of lines, and recoups the money from its customers in the rate they charge for their adsl/VOIP package. But Orange is still ultimately responsible for installing and maintaining the lines so if there's a problem, I contact Free and Free contacts Orange and Orange fix it. So it might take me longer to get my problem fixed that if I was a direct customer of Orange, because there are more links in the chain. I believe most parts of France are "dégroupé" now but maybe not all, and if your location is not dégroupé then you have to pay line rental to Orange, you don't have the option. If you are dégroupé then you do still have the option of renting a separate phone line if you want.
That's probably a bit garbled cos I'm not a techie, but I think it's rougly right.