Slight correction, police usually also means something bad has happened. Usually an accident that's already over and now needs processing, a robbery which you can only call after (most robbers don't say "go ahead and call the police, I'm about to mug you), murder victim has been found, murder was witnessed, shots fired heard in the distance, etc. Most of the time they can only show up after, take a report, maybe do a little investigating, and maybe find the guy wherever he went off to.

Sometimes you can call before, like if someone is kicking in your door and it takes a few swings or someone is having a suicidal crisis and is working up the nerve or whatever, technically hitting a silent alarm for a bank robbery would be "is happening" but we'll count it, but even then it still takes anywhere from 11min-30min for them to show up depending on where you are (last time I called it was over an hour in a city, after the event had happened,) and sometimes they don't even come until the next day in the sticks I visit frequently.

Police aren't actually that effective at *stopping" crime unless they just so happen to already be in the room when it happens (and that's assuming they're not the ones committing the crime themselves, which they also do more than anyone is comfortable with.)