says it might help RFI from AM radio stations (and maybe ham radio stations).
"Reasons for Using DSL Phoneline Filters
If you have a home phoneline network adapter (HomePNA) connected to your standard telephone wiring, 2Wire DSL phoneline filters preserve the quality and clarity of telephone conversations. The DSL filters help whether you have DSL, HomePNA, or both. There are also instances where AM radio stations interfere with telephone conversations; the 2Wire DSL filter also clears up this disturbance as well.
Looking at the guts of one of these, the coils L1 and L2 are on the same ferrite core, and L3 and L4 are on another ferrite core. Of course it matters which way the coils are phased, but if in the right way, they should do common mode rejection. Anyway, it would be quick to see if these filters would work or not.