In article ,
David Nebenzahl wrote:

Client has a bunch of phones in their house. Starting a few days ago,
several of them don't work; pick them up, no dial tone. (Confirmed with
a good set which I used to test all the jacks.)

But get this: while I was there, at least one of the phones that doesn't
work (a wireless phone) rang on an incoming call. WTF?!?!? It rang, but
when picked up--nothing, dead. No dial tone trying to make a call.

Anyone familiar with the inner mysteries of the telephone system care to
try to 'splain this? How could a phone not work, but still ring on an
incoming call?

That sounds to me as if the circuit has developed a "DC open", which
still has some small amount of capacitive coupling through it.

The central office won't "see" that the phones have gone off-hook, and
initiate a dial tone, unless the phone draws at least a few milliamps
of DC current from the line. If corrosion or a wire break has
interrupted one side or the other of the line, the phone wouldn't be
able to draw DC current through it, and wouldn't trigger the off-hook
detector at the central office.

Ringing is a different issue, as it involved higher voltages, AC (or
pulsed-DC), and is handled differently by different sorts of phones.
The ringing signal here in the U.S. is somewhere around 100 volts at
20 Hz, if I recall correctly. Real old-fashioned phones with real
ringers have an electromagnet/solenoid ringer, coupled to the line via
a large-value capacitor (agian, if I recall correctly) and they
require a significant amount of ringer current to operate.

Phones with electronic ringers... and wireless phones in particular...
operate differently. They really don't need to draw much current from
the ringing line in order to detect a ring... all a wireless-phone
base station may need to do is monitor the phone-line voltage with a
high-impedance voltage sensor, and "tell" the handsets to ring when it
sees a high voltage.

My guess is that: (1) you've got a broken wire or corroded connection
in the path which feeds the phones that no longer work, and (2)
there's enough leakage through the break to allow the wireless handset
to detect the high-voltage AC which means "ringing". Hence, the
wireless handset is told to ring. However, when you try to answer the
call, the wireless station base tries to connect to the line and start
drawing the normal amount of loop current... and it can't do so
because of the near-total break in the wire.

Try using one of those red-and-green-LED "phone line voltage and
polarity" detectors. I suspect you'll find that it doesn't light
(either color) when plugged into one of the dead outlets, but that if
you phone the house number from outside the LED will flash at least
slightly due to a small amount of AC leakage past the broken connection.

Start looking for bad splices, broken wires in junction boxes, or
places where rats or mice have chewed on the wiring.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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