NOMOBROBO also lets you add numbers to their block list. Depending on your network provider, it's not going to silence them completely. What happens is that incoming calls ring simultaneously on your phone and on the NOMOROBO computer. (simultaneous ring is a feature of IP telephony, and even then, many carriers don't offer it) If the computer recognizes the callerID as a spammer, it takes the call and essentially hangs up on them. Your phone will ring once, unless you are on a system that suppresses the first ring.

Because it's dependent on callerID, some spammers still get through by constantly changing numbers. But the annoyance factor will go way down.

The National Do Not Call is extremely effective at eliminating solicitation calls from legitimate companies. So it has some value. It won't stop political messages, community service messages, or certain types of opinion surveys. But it's the solicitation calls that are most annoying. The solicitations you get nowadays aren't legit...try asking what company they are with, what their address is, what their name is. They won't tell you, because what they are doing is illegal. If you could ever get their contact info, you can actually sue them in small claims court.