As to emergency alerting in the USA:

Public Safety and Homeland Security Bureau. (2018). PUBLIC SAFETY AND HOMELAND SECURITY BUREAU RELEASES ITS INITIAL FINDINGS REGARDING THE 2018 NATIONWIDE TESTS OF WIRELESS EMERGENCY ALERTS AND THE EMERGENCY ALERT SYSTEM (Public Notice No. DA 18-1294) (p. 5). Washington, D.C.: Federal Communications Commission. Retrieved from https://docs.fcc.gov/public/attachments/DA-18-1294A1.pdf