Sprint is the first carrier to enable opt-in wireless emergency alerts in cooperation with the U.S. Federal Emergency Management Agency (FEMA), and the alert service can already be activated on certain Sprint phones. The alert service is designed to send 90-character warning messages to mobile phones accompanied by a unique vibration and audio tone, and messages can be sent in crisis situations from state or local authorities, various federal agencies, or even the president of the United States — Sprint plans on joining with the New York Office of Emergency Management and U.S. Department of Homeland Security to test the service in New York City's five boroughs later this year.
You can activate the alerts on your own Sprint phone by dialing ##CMAS# or ##2627#, and choosing to "enable the CMAS client" when prompted. Note that the service is only available on thirteen of Sprint's devices — you can see a list of supported devices below:
Supported devices: HTC Evo Design 4G, HTC Evo 3D, Kyocera Brio, Kyocera Duracore, Kyocera DuraMax, Kyocera Milano, LG Marquee, Samsung Galaxy S II, Epic 4G Touch, Samsung Trender, Samsung Transform Ultra, Sanyo Innuendo by Kyocera and Sanyo Vero by Kyocera.