To investigate whether a fire call is real, wouldn't you need to be a trained firefighter? For example, you may have to enter a building that's possibly on fire to see if there's a fire. And a small fire isn't necessarily obvious by just looking at the building from the outside.

Also, if someone has to investigate before the fire department was dispatched, you'd lose a lot of valuable time, allowing the fire to spread. It could be a win if there was no fire, but a huge loss (burned buildings, dead people) if there actually was a fire.