What you are referring to is called WEA (Wireless Emergency Alerts, used to be called CMAS), which originates from FEMA based on an FCC spec. It doesn't work of

What you are referring to is called WEA (Wireless Emergency Alerts, used to be called CMAS), which originates from FEMA based on an FCC spec. It doesn't work off SMS, it's a separate protocol that basically delivers alerts that are affecting the area of the cell towers you are connected to at that time. The system had a lot of issues when it was first rolled out but it's been improved and seems to work well these days, although there are still carriers/towers that don't support it yet. They are actually working on expanding it with many additional alert types and increasing the amount of content an alert can contain. However, I have a feeling this will lead to even more people turning them off for good.