Calls repeatedly..at least twice a day and leaves no message.  I refuse to answer when the caller is 'Unknown', 'Restricted', 'No Name' because it's usually a telemarketer.  I figure that if they need to speak with me they will at the very least leave a message.
I don't think the "Do Not Call' registry really helps too much.

4 repeat calls within 15 minutes.  When I listened to the voice mail messages, they all consisted of lame music, no message.  I agree with the previous post, and also feel that the concept behind the "Do Not Call Registry' needs to be revisited, taking into account both the successes and failures, "DNCR" version 2.0 developed, and made available to consumers.