NEW YORK—Despite their best efforts, creative executives at the Young & Rubicam advertising agency cannot recall how Carrot Top came to be hired for the "1-800-CALL-ATT" collect-call campaign. "I really, truly don't remember ever casting that guy," Young & Rubicam creative director Molly Herbert said of the hideous, clown-faced comedian. "To be honest, all anyone here can remember is firing David Arquette two years ago, then suddenly there's Carrot Top on the set." Herbert and her team were equally at a loss over who developed the "Just Dial Down The Middle" concept.