Be aware that the do not call registry has 4 major loopholes:
*It does not apply to political calls.
*It does not apply to charities.
*It does not apply to any business where you have a prior relationship
*It can't be enforced on an entity based outside the US.

The latter was a major problem for me when a whacko Canadian religious organization kept calling us late at night. I spent a lot of time tracking down an e-address for their leader, then e-mailed him asking him to remove us from his call list as my wife was recovering from cancer and needed her rest. He did the honorable thing and the calls stopped.

A lot of the spam calls are from small local businesses (roofers, etc.) who seem unaware the registry exists. I've read of people who capture illegal calls via caller ID, then blackmail the illegal callers for a fee to avoid being reported.

When we moved here I only gave my cell phone number to entities I wanted to hear from. My land line gets lots of calls and I let them go to voice mail. 95% of the callers leave no message. 4% are spam calls which do leave a voice mail, usually campaign auto callers. This is the best system I've found.