I was told by another Verizon employee years ago that every working telephone jack other than the one feeding the DSL modem should have a DSL filter on it, even if no telephone is connected to the jack.
Thinking about that I would initially disagree (because no phone there to mess with the DSL) but thinking deeper, an unused line would create a possible open loop that may cause a variation of "tv ghosting" which occurs in exactly the same circumstances, which could cause problems for the DSL.
I have a much better solution at my house. I intercept the phone line as it comes in the house, through a splitter, and one split immediately through a filter, to the line that feeds all the phones in my house. The other naked split goes straight to the modem.
