The only I reason I can think of is that they've had that number ready (and it has to be the most heavily-bugged phone line in the world right now) since they first found the note and nobody has called.
Maybe they didn't find the note until this morning. That's entirely possible. But nobody has called all day, and Moose felt necessary to prod the shooter.
The sniper does feel invincible and by essentially challenging him to call, it might well happen.
This perp has been changing his M.O. with each media description thereof.
If the police expect him to call, he probably won't.