(KMOV.com) -- Tuesday night, a shrill alarm from Bella Winters' phone woke her up.
"It's scary. You hear it and you think, 'What in the world is going on?'" said Winters.
It was an AMBER Alert issued from Nacagdoches, Texas. But, others with smart phones may not have received one at all.
The answer is that not all smart phones are Wireless Emergency Alert capable. You can find out if your device is WEA capable here: http://www.ctia.org/your-wireless-life/consumer-tips/wireless-emergency-alerts
Others saw delays in the alerts, receiving them later than others in their households (even if they are on the same wireless carrier). The reason may be that the alerts are designed to avoid interrupting a data session or voice call.
Sometimes, even if you are in the same location as another person with a phone who received an alert, your mobile devices may be using different towers.
The alerts are meant to target specific geographic regions. As you move around and your phone begins to use a tower in an alert zone, you should receive the WEA. That may explain why someone in the metro-east may not receive an alert issued for Missouri, until crossing the river into an alert zone.
The Wireless Emergency Alert system went into effect January 1, 2013, but are becoming more common as more devices are becoming WEA capable. You don't opt into the alerts, most WEA alerts to newer devices are automatic as long as your wireless provider participates in the program (most do).
Verizon offers information about WEA on its devices here.
If you're an AT&T customer, you can read about WEA here.
The WEA isn't just used for AMBER Alerts, it's meant to be used by certain agencies to report a national disaster or a weather emergency. The system may also be used for a Presidential alert, which is the only alert you can not turn off in your settings.
"You can turn AMBER Alerts on or off or turn severe weather on or off... with the hopes that people would leave them on," said Mike Lovasco - a district manager with Verizon Wireless.
"For me, it would be worth waking me up if it meant that someone else received that message and saw the car and saved a child's life," said Winters.