These are government-issued safety messages that include AMBER Alerts, emergency alerts – such as man-made or natural disasters, and Presidential alerts. Wireless Emergency Alerts are part of the FCC’s CMAS program and are mandated by law. You may turn off alerts (except for Presidential alerts) if you choose. Go to Settings >Notifications>Turn On/Off.
When the software update is delivered to your phone, you will see a message that states “Carrier Settings Update: new settings required for your device have been installed”. After that, your iPhone 5 or 4S will receive all Wireless Emergency Alerts by default.
AT&T notes that while that software update is mandatory you will not be charged for the data used to deliver it and you will not be charged for Wireless Emergency Alert messages. Users on iOS 6.1 or higher will receive the update.