[areafiftyone]

If the sniper gave them a telephone number, why don't they just call him/her?

I think the number might be a phone booth or something like that. A public number so the police cannot trace him. I think he is just teasing. But the idea that he has left notes or tarot cards to the police lets you know that this person is psychotic and is in charge right now. He is going to play games until he gets caught.

[JameRetief]

Come on people, the number that was left was an unrelated third party number (probably a pay phone) that the shooter would call to talk to the police. The phone company has "confiscated" the number and it now redirects to a dedicated police line. The shooter did this so that now the police know for certain that when he calls them on that line that it is indeed the shooter and not another nutcase claiming credit.