I had a visit from Dutch detectives when I lived in Holland , I had a bit of a reputation and they knew a bit about me but nothing they had any evidence of, they asked me a lot of questions about a guy who I had done some business with .
At first it was friendly , friendly but I didn't really know the guy that well and when they weren't getting the answers they wanted they threatened to have my work permit cancelled. I told them they could do what they want as I don't know anything that could be of use to them.
They let it drop and thanked me for my time , I asked "Why the interest in this guy ?" and they told me he was selling all sorts of shit outside local schools.
I would've given them anything they needed if I had known anything but unfortunately I couldn't sell him down the river.
Some fuckers deserve to get arrested, I have done time and deserved it .
It would depend heavily on what kind of crime was being commited wether I dial 999 or 112 or even 911 or whatever the emergency number is where I'm living at that time.