Called the number back and a voice recording plays saying they are Home Energy Solutions. Something about providing solutions to making your home green. It says there is no need to call them back and apologizes for disturbing, but no way to get off their call list. Just block it.
Solar panel sales, one of the few bad things about living in California
"They called and hung up."
Actually, it was an auto-dialer. In one of these greasy boiler-rooms they have an auto-dialer which dials five numbers at the same time. The first mammal that picks up the phone gets routed to the operator; all the other four calls are the familiar "nobody's there". A computer hung up, not a human. This might be helpful for the other 10 calls you get during the day. If you pick it up quick enough, you'll be the first in line. Then you can listen to their blabber, tell them no, hang up and likely get a call from them at a later date.
If it's a really cheap operation, you won't get crossed off, because they don't care if their operators waste time. If it's a marginally cheap operation they will strike your number from the list and you won't get another call. Flip a coin to find out which it is.