If you receive two filters, as i did with Wanadoo. Then Plug in filter one in to the wall socket in the hall.
Run phone cable upstairs(as you already have)
Now... assuming you have a socket in to which you have your extension phone connected. Plug the second filter into this socket.
You now have two connections to make in the filter, one for your phone and one for your BB Modem.
It is now that you will need the RJ11 TO RJ11 cable to connect from the filter to the BB Modem.
Hopefully this should save running more than one cable up the 'apples and pears'