Uhhhhh, what exactly do you mean by " (this was some time before I got into security systems professionally)." ?
Do you install alarm systems now? For a living? For how long? ......and do you mind my asking what kind of systems you install? Did you apprentice with anyone? Do you know what line seizure is? Do you know what an RJ31X jack is?
If the perp was going to knock the phone off the hook, how would a system that " plugs into a phone line and power outlet" get to call out in the first place? If the operator heard nothing, while listening in ...... then no dispatch would be made? But .... If they still would dispatch ..... wouldn't that be a false alarm too? So how is this different than an alarm that goes through a central station without .... listening in?
Suppose there was an intruder there but he was upstairs ransacking the bedroom but the operator didn't hear him and the operator doesn't call PD because they didn't hear anything?
Exactly how long do you think an operator will sit and listen since the central station line that they are listening on is a line that cannot be used for other incomming alarm call all the while they are listening? And that operator can't be handling other alarm signals while listening in. How many operators do you think will be listening in on false alarm calls, while blocking other incoming alarm calls that may be legitimate? How efficient and / or accurate or more reliable, than standard central station monitoring could this be?
Actually this may clear up something else though. If the policeman DID say that they believe this kind of system is more reliable than other sytems, it makes it more understandable why they also aren't smart enought to realize that there really isn't a 99% false alarm rate.