Last we heard of the federal government's Wireless Emergency Alert system, only Sprint had signed on to deliver the SMS warnings. Now, with the secured participation of all four major carriers and smaller regional operators, that gratis service is set to go live this month, covering nearly 97 percent of active mobile users. Using a "point-to-multipoint system" that targets at-risk subscribers, the National Weather Service, FEMA, FCC and Department of Homeland Security-backed initiative works by sending location-based messages of 90 characters or less to nearby handsets in the event of an imminent meteorological threat. The mostly opt-out service will also accommodate AMBER and Presidential alerts, although you won't have that flexibility for missives sent from our head of state. So, the next time your phone gives off a strange auditory tone, you'll know to head for shelter.