It's a long time ago now but if I remember correctly they used the constant "All Clear" siren to call out the fire brigade in my home town. Most of the firemen (they were all men in those days) were retained part-timers & it was the most effective way of calling them out before the pager was in general use. I expect they use mobile phones now.
...ours used the slow "Warble" sound for some strange reason, which was, of course, why I found it strange, and worrying at the time!
It could be heard all over the City, and the surrounding area.
Paul.