A 9-foot-tall, narrow structure installed this past week on a Manhattan sidewalk is signalling a plan to turn payphones into what's billed as the world's biggest and fastest municipal Wi-Fi network. The first of at least 7,500 planned hot spots are due to go online early next year, promising superfast and free Wi-Fi service, new street phones with free calling, ports to charge personal phones and a no-cost windfall for the city.