wrote
Owain wrote
wrote

Its completely trivial to monitor the loss of the
50VDC that shows the phone line has been cut.

Doesn't a mildly intelligent criminal (if there are any)
simply short the two wires of the phone line, then
it's permanently 'on hook' rather than broken.

Then there wouldn't be 50 V across the pair, as there wouldn't be any
p.d.

As in every phone line in the country that's not in use.

But only a fool would have the alarm dialler on one of those lines.

And plenty of systems leave the line connected
and active when its not in use, essentially so
someone who wants a new phone service can
just pick up the handset and talk to a sales
droid to have the new phone service enabled.