I guess that's probably why I get a lot of hang-ups when I answer unknown calls at work, I say more than just 'Hello?' since I'm answering for a business, and I usually hear a click after a second and a dead line. Unfortunately, I don't have the option to just ignore them, since some of our clients and even an employee call from unknown numbers.
I always tell people calling from phone companies that we don't have a phone. They never quite know what to say to that. Other annoying salesmen get directed to the warehouse cat, who never seems available for a phone call.
The main problem I see, even with the do not call list, is enforcement of it. When I answer without looking at the caller ID first, and it's a telemarketer, they just hang up when you ask for their information, so it's hard to turn them in. I suppose I could play along to get their info, but hanging up is just easier.