As other posters have diad - one ring and hang up - when you call back you get" HAI! THIS IS JULIE WITH REWARD REDEMPTION offering a $100 REBATE VOUCHER." .
I suspect they do this to somehow sidestep not adhering to the national DNC database. by callign and hanging up after 1 ring and you calling back to see WTF is was, now you're CALLING THEM.