As it happens I paid my annual bill to ADT this morning. It was £329. For this the system is monitored. If the alarm is triggered, whether by an intruder or, say, the cat getting into the sitting room when the alarm is on, or we have gone out into the garden without switching it off, the alarm rings (very loudly). You have 30 seconds to dash to the control box to enter your number and switch it off. If you do not do this, or if there is an intruder, the alarm continues and ADT ring you. if you know the reason for the activation you give a password and they do not summon the police. If no reply and no password, they call the police.
My husband works away and I am alone in a big house. I cannot hear the downstairs doors being opened when I am in the bedroom. Because I always set the alarm I am quite happy and never frightened. However we do live fairly close to the town centre and police response is very quick. If I lived in the country where the police would take ages to arrive I am not so sure it would be much help.
If the panic button by the bed is pressed ADT don't bother checking, they call the police immediately. Very embarrassing when it turned out a new kitten had trodden on it.
You do need to get into the habit of setting it every time you leave the house, even if going just up the road, and it can be difficult to find five willing key holders living within 20 minutes drive.
ADT routinely check the system twice a year; the annual fee covers replacing anything faulty, and it has a beneficial impact on our house insurance premiums.
Worth every penny as I said to the young man who relieved me of my £329 this morning!