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Here's the reality: Even if you're lucky enough to live in an area where cops respond to alarms (and we'll have more on that in a moment), they still may not respond quickly enough. The alarm doesn't summon police directly, you see -- it alerts our call center. Then, someone at that center phones you. He or she waits for you to pick up so they can ask if you're aware that the alarm is going off. The company only goes to the next step if you say that something's wrong, give an incorrect verbal password (pre-established to make sure you're not a burglar in a wig), or fail to answer the phone.
Often, that next step is calling a second number. Many police departments force alarm companies to try two numbers to reach the homeowner, a consequence of these systems generating so many false alarms (and I'll have more to say about that, too). The cops don't want to send out a car unless they're really sure. Florida, Georgia, Tennessee, and Alabama each have a statewide rule for this. If you can't provide a second number (because, say, you live alone and don't know your immediate neighbors), the police won't come, period. Those lonely hermits with no friends and only one number to call? Too bad! Any good hermit should have booby-trapped their front door with a dangling shotgun.*
*Do not plant booby traps around your home