The number showing up on your caller ID is not the number they are calling from. Programs can be downloaded from the internet that can make it appear that the call is coming from any number the caller wishes to use. Therefore, it is of no use to report it as a violation of the Do Not Call list. The call is untraceable and the caller knows it. He/she is not going to abide by the Do Not Call list. They are not going to be caught.
Any caller who does not abide by the Do Not Call list should be considered a probable scam artist, "phisher" or looking for "live ones" - suckers who will answer their phones. If you do not recognize the caller on your caller ID, DON"T ANSWER!!! If it is a legitimate call or really important, they will leave a message. If you answer, you are a "live one". I quit answering these calls several months ago, and the number I've been getting has started to drop off very markedly. I do check the numbers that come up on caller ID by Googling them, just to be sure.
On my cell phone, when I get one of these calls, I assign it a name - DO NOT ANSWER - and a silent ringtone. When my cell phone shows that Do Not Answer is calling me, I don't!