Gary Barnett was sitting in his Manhattan office one morning in the fall when his old-fashioned flip phone started to buzz. On the line was a real-estate agent who was marketing the New York developer’s latest Condo project, a soaring 1,550-foot tall building known as Central Park Tower. With a total projected sellout of more than $4 billion, the skyscraper is the country’s priciest-ever condominium project and, when complete, will be the Tallest residential building in the world.