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Surely there will be alarm logs to prove it the alarm was active, and the correct zones enabled at the time?
If it was active, and it failed to alert the lines had been cut (do those alarms do that?), then it failed to pick up the broken glass via glass sensor, then all the internal motion sensors failed, I would see why the owner is justifiable peeved off at ADT.....
Some alarm panels allow you to set phone line tamper, we rarely use it down here though since the phone company has their own issues, and it means constant signals to the CMS, same goes with low batter restores.
Anyway, if they cut the phone line, and there was a back up to that, it would be a radio system which would send the data to the CMS. Now that the home owner's phone line is down, there is no way for the CMS to alert the home owner of anything. They could call the cops or a security service and have them goto the location, but by that time the home owner could be dead.
In this case it did not mention anything about radio backup, only that some form of backup was supposed to alert the home owner. I imagine they meant tying in the phone line tamper to the siren, but this is also not mentioned, and its not a backup per say.
As to Glass Break Detectors, unless they have the sensors tested occasionally, there is no guarantee that they are working. That would need to be an additional service and it was not mentioned either. They have Glass Break Detector testers just for that.
Another thing to note that we are not told whether the system is wireless. In this case its possible that the CMS had low battery signals and alerted the home owners, and the home owners never had the batteries replaced - this is common due to the expense of both the battery itself, and the service for those that dont want to do it themselves; which is the majority of home owners (at least down here).
Bottom line is noone should depend on an alarm system, or any other security system, to ultimately protect their life. An alarm system is essentially designed to make an alarm sound when there is an incident, but that will greatly depend on what alarm devices are installed. What happens then is up to the home owner, or if they are lucky, the cops.
Personally I live with Iron bars on my windows and doors, but even that is only a deterrent and may slow them down. That along with weapons, alarm, and cameras, are all just tools that will help, but do not guarantee anything at all.
Put the blame where it rests, on the criminal that killed the victim.