This is apparently a robo-call., a computer-generated automatic dialler. they never answer or leave a message;. it should be illegal. as they invade one's privacy.
These guys call periodically. Called today (9/3/14). I picked up the phone. I said nothing. I hear nothing on the other end for about 15 seconds then they hung up. I said hello once (stupidly) and got the spiel about a $100 wally world gift card. These guys really peeve me, and I know the only way I CAN hurt them - WASTE THEIR TIME! I asked questions for at least 15 minutes before the guy hung up on me. I didn't get any more calls from these guys for another six months. What a pain.