[daltore]

For reference in case it helps, there is a high-frequency signal for DSL and a low-frequency POTS (plain ol' telephone signal) on the one line coming from AT&T. The filter separates the two with high- and low-pass filters so that each device only gets the signal frequency range it's meant to. The filter should completely eliminate the possibility of the modem getting a POTS signal (the modem should have its own high-pass filter) or the phone getting the DSL signal.

Have you tried a different modem there? You can usually find them at Goodwill or such places. Might be worth a try. On the other hand, if you've got a DSL connection at home, you could try using your modem with the pantry's setup just for a test.