Could this be a short (through defective wiring, lightning arrestor, or perhaps through a defective DSL filter, since those were probably installed recently) that only conducts at the higher voltages involved with the ringing signal?
I don't think the poster has said what happens with a hard-wired phone (best of all with an old one with the mechanical ringer). Is there any evidence that he's getting (adequate) ringing voltage under any conditions?
There are others here who would know far better than I, but when DSL was hooked up, would his telephone line been switched to a different card at the office (or in the pedestal on the street)?
Dave
[TELECOM Digest Editor's Note: Those were going to be some additional questions of mine: perhaps the original writer will respond to these items as well. PAT]