believe this is netpro call center: donation, survey and political. put 720-452-5xxx and 720-452-6xxx in block list and life will be better.

Says "I" on ID. We've had this number for years.
They don't leave a message.

Caller ID said "I" and sounded like recording. The recorded voice was male and said "Hello I am with Colorado Police .... something! then I herd clicks and the call disconnected. I was very worried until I found these other comments about the phone number.

Just submitted a complaint on the do not call registry.  https://complaints.donotcall.gov/complaint/complaintcheck.aspx

What 90% of these types of calls are people, companies, countries hoping the number they are calling is also connected to your computer.  If it is, they stay on line long enough to be able to get into your files and then they can get all your personal information.  Full name, DOB, SS#, address, etc.  If you don't know the number you should never answer it.  You can always call your family & friends and ask if they just called.