In order for users to receive text alerts, you will want to create a News or Announcement category specifically for emergencies. 

Go to onMessage:
  1. Select Content
  2. Select Announcements or News
  3. Click + Add New Category
  4. Mark Allow for Email/Text Notifications
  5. Click Save
Note: This can also be done by going to Core > Communication > Content Types and then following steps 2-5

Users will not be subscribed to the new emergency category automatically. In order for a user to subscribe and receive text messages they must follow the steps listed in this help topic.