I've recently had a few downs and ups, experience wise with DSL filters and thought that I'd share what I'd learned. Basically, the basic low end ones don't eliminate much of the audible line noise/RF interference. I've been working with DSL connections, over AT&T lines (formerly SBC, Hey! What ever happened to the anti-trust/AT&T/Bell Telephone/break-up ?!?!) and Earthlink.
Earthlink is generally alright to deal with, not great. However, an AT&T employee/friend of the family clued me in the that the filters that they sent with the installation kit is sub-par. Seeing what he installed, I contacted the web-store from that company (Excelsus), and bought the even higher end, RFI elimating version of the filters, installed them, and things cleared right up, and the voice signal even seemed to be stronger than before any of this installation activity. The model of the filters that I went for are Z-275P2J, and I strongly recommend it. BTW I'm running a wireless LAN at 1.2 Mhz and some wireless phones at the same frequency which I understand can cause some interference, but the new filters took care of that issue.