When you call the number back, you are immediately routed to a recorded call explaining how you can enter your phone number into their internal 'do not call database.' I have down this on more than one occasion, and still continue to receive calls from this spammer.
The message was . . . you can lower your energy costs with new windows, at that point I hung up.
Because they did not provide a call back number in message they left (I saw this number on caller id) they are not be trusted in my opinion.
Canned scammer recording about energy savings on new windows.
No name on caller-id. No voice message. No chance I will pick up a call from an unknown caller.