Why am I not getting the national emergency alerts?

I'm wondering why I have never gotten one of the national emergency alerts that supposedly come to your phone automatically. Tonight there were several tornado warnings in my area and everyone around me was getting the national alerts, but I never received it. The messages weren't coming through an app and no one had signed up for them. I'm wondering how I can start getting them. I have an Iphone 4 with Sprint as my service provider. Does Sprint not offer this service or am I doing something wrong? Thank you for your help! Google has failed me tonight.

Sprint has the service along with AT&T and Verizon. The phone must be compatible with the service, and the iPhone 4 isn't.