Broadway Producer Hillard Elkins was strolling beside Manhattan's Central Park last week when he got an itch to speak with the man directing the revival of his musical Golden Boy, in rehearsal at the George Abbott Theater some 20 blocks away. Elkins sat down on the nearest bench, opened his briefcase and picked up the telephone. "Hello, mobile. Come in, please. This is JL 5-5035," he began, and in moments the director was on the other end of the line.

Elkins is one of 30-odd men so far who have bought Portatronic Systems Inc.'s new 19-lb. Portable Executive Telephone (PET),...