Life Insurance telemarketing leads.  They call and leave their caller ID and hope you will return the call, upon which you get their recording.  They spend a good deal of time talking about how they comply with the law.

Interestingly, their phone number shows up for the Del Mar Parkway Neighborhood Assn in Aurora, CO.  The contact seems to be Chris Baca or christine_baca@hotmail.com.  All public information.
Go to: http://www.americantowns.com/co/aurora/venues ... annual-clean-up