Has called several times, but never leaves a message.

These a$$holes call and call and call. Really, what could they possibly want.

Several things, and finding out when a live voice is there to answer the call is up there. They might not steal personal or financial information, money, or break into your computer, but getting a number that has an answer, especially if it's a live voice, will work, too. You can ruin all of them by not answering in any way if possible. Times when somebody manually takes the call could be charted and passed along so other pests know when to call.

Number blocked.

Has called several times, but never leaves a message.