Hmm...well then perhaps yours was a filter/splitter combination. Perhaps the DSL side wasn't actually filtered. It's not possible to successfully use a filter on the DSL line, so that must be it.
Yea, to add to what Gary said, AT&T ships out their DSL kits with filter/splitter combos. One side says Phone (the filtered side) where you plug the phone, and the other says DSL (that's not filtered) where you plug the modem.
Indeed, I do use about three of those single filters that you pointed out, Gary.
In that case, yes, you would have to use a seperate splitter. But I guess that's not the case with Steve, who has the filter/splitter combo.