Never call them back! If they are a legitimate company, they will leave a message. You have nothing to gain by calling back. All you are doing is confirming they reached a valid number. Or, they may have spoofed the number and you could be calling an innocent party.
Phone rang twice, then hung up. From the looks of other like comments, it's not likely a raging success of an enterprise, whatever the scam is. They likely need to say something to make any progress, you think? Idiots.