Depends on your area. Calling 911 because someone rang your doorbell at 2 AM would be an overreaction, IMO. Calling 911 because someone is pounding on your door trying to get in, different story. In either case, I'd grab a gun and light and look through my cameras before doing anything else.
Seriously guys, most of y'all spend hundreds if not thousands of dollars on your toys every year. Why not spend $100 on a wireless camera that hooks to your computer? What, you're a total Luddite? Ok, spend $100 on a wireless baby monitor camera aimed at your door, instead.