My undestanding is that the FXO port holds the line that comes directly from the carrier, which would be a regular POTS line.
It would plug into a card or module in the back of a router with an rj11.
If this is correct, why would you need it to go to your FXO port on the router?
Why not just plug the line directly into the analog device that will be using it?
What benefit do you get from this?