iOS 11 has a new Emergency SOS feature which calls the emergency services fast.
To activate it, simply press the power button on your device five times. This brings up a screen that displays the slide to power off option, but also a new slider for the user’s Medical ID, and another for Emergency SOS.
Slide this and your device will call the emergency services.
It’s worth noting that activating Emergency SOS deactivates Touch ID on next login, requiring the user’s passcode instead.