I'm re-routing and adding some phone jacks in the house and found an oddity.

Originally, the line came from the telco side of the demarc box over to my
side and was screwed down onto two terminals (red/green). The two cables
that feed the outlets in the house were tied down to these terminals as well
(white/blue pairs).

It appears that when the alarm system was installed, the installer lifted
the original cables and ran a new cable from the telco box to the alarm
control panel. The telco screw downs are tied to his cable (white/blue)
only and the pair goes straight down to the alarm box. It comes out of the
alarm box on his (white/orange) second pair back to the telco box and is
spliced to my original cables' (white/blue) pair. I'm guessing that if/when
the alarm box grabs the line on the inbound pair, it has the outbound pair
open so another phone can't come off hook.

If found the installation manual on the alarm box and what was done is
exactly "per the book". It tells the installer that the alarm system should
have the "first" shot at the phone line and that nothing should be between
the alarm box's connection and the telco interface. No way to fault the
installer, but I do have a couple of concerns.

Since the alarm was installed, we have added DSL. And in case anyone is
particularly knowledgeable, the alarm box is a DSC PC1555.

The alarm guy's wiring is not CAT5, it's CAT3 or some other kind of station
wire and because of the routing, the circuit runs through it and the alarm
box before coming back to the telco box where it's spliced to my inside
wiring. That's adding about 100' of lesser quality cable before I ever have
a chance to get it to my modem.

The DSL signals are high frequency, so that "noise" is getting shot straight
into the alarm box and I see no kind of filter outside the box and no
reference in the box's manual about it having any kind of high frequency
filter.

Occasionally, we have problems with the DSL and I wonder if the alarm box
occasionally grabs the line to report in or just test itself. There's no
sort of filter around the alarm box for this either. If the alarm box comes
off hook, it has to "look" like a telephone and therefore it's presenting a
600 ohm load to the line across the entire frequency band. It think that
this is what the the "store-bought" DSL filters are supposed to prevent.
Maybe this is happening and causing out intermittent "outages". If the
alarm box is grabbing the line and doing what I suspect, then the house
outlets are effectively "dead" when it happens.

Has anyone else ran into this before and did it cause any problems?

My first thought is to tap the alarm guy's cable and grab what is
effectively the "first" presence of the telco pair inside the house to run
my additional outlets and put the appropriate DSL filters on the new drops
that I run. Secondly, I'd like to find out if there is any need for some
other kind of high frequency filter between the line and the alarm box.

TIA-trebor