It is built-in, broadcasted by the cell network itself: https://en.wikipedia.org/wiki/Cell_Broadcast https://en.wikipedia.org/wiki/Cell_Broadcast https://en.w

It is built-in, broadcasted by the cell network itself: https://en.wikipedia.org/wiki/Cell_Broadcast https://en.wikipedia.org/wiki/Cell_Broadcast https://en.wikipedia.org/wiki/Common_Alerting_Protocol https://en.wikipedia.org/wiki/Common_Alerting_Protocol