"everything else gets connected after the filter."
Sounds like the problem right there... Filtered and un-filtered connected somewhere. Good sign of that would be the DSL working OK, then if someone picks up a phone it drops out. They may even have called tech support on that same line....
Anyway, Hal is right, a single DSL filter and to the modem, network from there if need be. And the filtered voice also distributed from there.
And some may disagree about the need of additional cabling. But my prefered method (except for people who can handle having a remote modem) is 2-cat5 and 4-pair phone (One of which would be unfiltered DSL) to most locations. This way you can still have the modem next to a computer anywhere, and LAN back to a hub in a remote location, then back to the computer it may be next to.