Report it as a violation of the Do-Not-Call registry
https://complaints.donotcall.gov/complaint/complaintcheck.aspx
IIRC, they ask for the called & calling phone numbers, the entity doing the calling, the time of day and date.
While we're on it, this is also handy to know for unwanted text messages
Many carriers (such as AT&T[13], T-Mobile[14], Verizon, and Sprint in the US) allow subscribers to report spam by forwarding the spam messages to short code 7726 (spells SPAM on a traditional phone keypad) (33700 in France).
I agree with you in not even answering any of these 'unknowns'.
Just today, here are the phone numbers that called me. Don't have any idea who they are or from, but I'm going to start keeping track of all the numbers and if they start calling too much I'll just go to Verizon website and block the numbers from even calling me:
360-529-6130
630-240-0775
971-220-1262
Maybe if we all keep track of the numbers that call us and if they become a bother, just block them!