Got a call from this number, left no messages

I got a call from this number on 10:47am (PST) 11/07/2014. I don't answer any number I don't know. No message left.

Have been receiving calls from them for the past week now, I don't answer unknown calls if they left a message maybe I would call back, however after about 10 calls they still left no message.
I can't imagine it being debt collection seeing as I have no debt.

I called the number back and it said I have reached an unassigned number.

I received several calls from this caller as well, at intervals of 20 minutes for 5 times.  I finally answered the phone.  There was a silence delay at first, about 3 to 5 seconds so likely an automated dialer waiting for a person to pick up at the source.  It was some guy with a foreign accent, talking about saving me some money on my electric bill.  He sounded like he was leading into a sales pitch so I hung up on him.  Given this information, talking to this outfit or person (whatever it is) may be a waste of your time.