The test alerts can only be received on some LTE and WPA enabled phones. So if you have a an older phone that only has 4G, you will not receive the alert.

More info.
https://mobilesyrup.com/2018/04/06/lte-wpa-april-6-alert-ready/
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock