At nearly quarter to midnight on Friday, all 156 of Dallas' emergency alarm systems went off. It soon emerged that hackers had set off all 156 of the outdoor warning sirens. The Dallas Office of Emergency Management swiftly set to work turning all the alarms off before determining how to prevent further attacks. That meant some who weren't calling about the sirens couldn't get through to emergency services. The siren system is back online as of yesterday, according to a Dallas City Hall spokesperson.

Source: Forbes April 10, 2017 10:30 UTC