me hath wroth:

Or one of the filters has failed due to lightning (or some such)...

There's not much to fail inside. Inside a microfilter:
http://802.11junk.com/jeffl/pics/dsl/slides/microfilter.html
All passive parts.

The Siecor outdoor splitter is protected by zeners and two fuses on
the input lines:
http://802.11junk.com/jeffl/pics/dsl/slides/splitter.html

If it were hit by lightning, it would probably open the inductors,
when would then result in no dialtone.

Incidentally, the connection between the line and the "DSL" port is
straight through, with no components in the way (except perhaps the
fuse).

However, I've seen far more improperly installed microfilters than
defective filters. There's 3 ports on each filter, which offers 5
different ways to connect it, only one of which will work. Given the
opportunity and the odds, the inexperienced user will usually get it
wrong.

Much more common is installing the 3 port (line, DSL, phone) properly,
but coming back later and plugging a phone into the unused DSL port.
To solve that problem, SBC/AT&T now supplies one 3 port filter and 3ea
2 port filters, which can't have an extra phone plugged into the DSL
port (because there is no DSL port).


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558