Earlier quoted context omitted.
I wonder if the siren thing varies from location to location, a good friend of mine lived right next to one and it seemed you couldn't go 15 minutes without being drown out by them heading out.
In NJ they go off all the time to alert the volunteers not at the station, and it's used for all emergencies, not just fires that need trucks to roll out.
You will always see the flashing lights but they have never blasted the sirens at night.