Way back when I were a lad, an alarm system rang a bell went it thought there was a burglary, and everyone within earshot would panic, they’d call the police, the police would come out, arrest the criminal, and all would be well with the world.

The problem was that the systems weren’t 100% reliable. And every time there was a false alarm, the police would come out, not arrest a criminal, and they’d go away & grumble about it to Sarge.

So along came a bright spark and set up a Monitoring Service. Instead of ringing a bell at the police station, it would ring a bell, metaphorically speaking, at his building. He would wait a few minutes for the customer to phone him and say that it was a false alarm. If the customer didn’t do that, they’d phone the customer. And of course, they would charge a monthly fee for this service.

But then some customers said that they didn’t want to pay a monthly fee, thank you very much indeed – wasn’t there some way that the alarm system could phone them? So another bright spark came up with a way to do that.

So when people talk about “bells-only”, “fully monitored” and “self-monitored,” that’s what they mean.