ETA: it appears that the wires of the new jack were reversed. We'll see if fixing that helps . . . Thanks, all.
As I've mentioned before, we turned our garage into a library, complete with telephone jack (and a cable hookup, on the same faceplate). When a phone's plugged into the new jack, it works: it makes outgoing calls and receives incoming calls.
Today Chad moved his computer equipment downstairs, including the DSL modem and the wireless router. However, when the DSL modem is plugged into the new phone jack (either directly or through the unfiltered side of a splitter), the modem will not connect to the Internet, as signaled by its "ready" light just blinking and blinking and never going solid.
The modem still works when plugged in upstairs, where it used to live, which is how I'm posting this now. But this is not a good solution in the long run.
So, does this mean that our contractor did something subtly wrong in installing the new jack? Is there any useful way we can track down this problem? Any advice?