Related to this post in a way (Is the guy with the shotgun at the start of Dark Knight a regular comic character?), during the robbery, we see that one disables the alarm on the rooftop. Here is the quote taken from the screenplay:
Here comes the silent alarm.
(touches a button)
And there it goes. That's funny. It didn't dial out to 911- it was trying to reach a private number.
(Behind him, Happy RAISES his silenced HANDGUN.)
Is it a problem?
No, no. I'm done here.
Why a private number and not the police? I understand from the post linked above that mobsters control the bank, so is the phonecall to them? And if so why? It's not like they're going to rush in and save their money while the Feds will be coming in (which they do eventually, I'm guessing someone called them by phone since the alarm was off). Is there someone else to call apart from mobsters?