The North American universal emergency telephone number, 9-1-1. It was originally referred to as the "nine-eleven" service, but has long been exclusively called "nine-one-one" to emphasize the buttons pressed to activate it.
This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. If an article link referred you here, you might want to go back and fix it to point directly to the intended page.