Just received, won a home security system.  

I usually press one to get a live body on there and wait until they hang up.  I think it's best to waste their resources, hopefully it becomes more costly for them to do business if each call is answered but no one responds.

After my Hello, there were a few seconds of delay, when an automated male  voice, sternly said, "Do Not hang up!  Congratulations! ...."  At this point I hung up.

The National Do Not Call registry did at one point work as it was designed.  Now the public is being swamped with these useless calls and the FTC is attempting to take steps to curtail the robocalls.  Not fast enough!

Over at today.msnbc.com has an interesting article under Rossen Reports: Telemarketers Ignore Do Not Call List.  Yes they call even though it is illegal to do so once you are on the registry.  Since little effective action on the part of 'the powers that be' prove ineffective, the call just keep on commin'.

they called and then hang-up when I answer, I am on the national do not call list tell them to quit calling.

Same call received before...said,
"Do not hangup" as soon as I pick up on ring.  As in the past I've reported and blocked them, now calling from a different phone #.   I am so sick of these calls that even my phone has no more room to block them.

Called on 1/4/13 at 11:38 am.  Got answering machine as we don't answer calls we can't identify.