I hate to have to ask, but why is that funny?

Because it's a TV show. So first off, any phone number that appears on the show will be a 555 number. But this is a police trace, and Chief Wiggum doesn't know he's on a TV show, and a trace is an absolute way of finding where the person is calling from. The fact that he cancels it after the 555 is funny because it's not a fake number in their world.

That's the best I can explain it. I guess you might have to be there.

DiGNAN