JAMES REYNOLDS SCIENCE CORRESPONDENT AMATEUR radio hams are usually excited by the faint buzz of a distant shortwave station, but a group of scientists believe they have received a message from extra-terrestrials. Astronomers think that a signal picked up by a radio telescope last year shows the highest probability yet that ET’s family may have returned his call. In February 2003, scientists involved in the search for extra-terrestrial intelligence (SETI) pointed the huge radio telescope in Arecibo, Puerto Rico, at about 200 sections of the sky. Unexplained radio signals had been detected twice by the same telescope in these areas...