I did alarms professionally for a number of years.  Most reported by
phone line, with an RJ-31X connection that would allow them to take
over the line even if someone was in the middle of a call.  They also
had a sensor that would detect a line cut, though they could be fooled
by a 48V battery.  If the line cut sensor tripped, then we'd set off
the local alarm.   For our higher security systems, we did all that,
plus an RF link to some other client, so that an alarm on client X
also tripped a silent alarm on client Y, and vice versa.   You'd have
to have very tightly held inside information to get past that.   No
external antennas showing of course.

I did a lot of things with the systems to steer the bad guy into
making dumb mistakes, like leaving wires exposed where they would be
easy to cut, having very cheap and cheesy key switches, etc.  Those
apparent weak points are just invitations to trip the alarm sooner,
before entry is made.

In the end, only physical security can protect you from a "smash and
grab" though.
--=20
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