Apple’s iOS 11 lets you discreetly call for help in an emergency. If you are feeling threatened or are incapacitated, just quickly press the sleep/wake button on the side five times. Your phone will auto-call your local emergency services, and can also send a notification to a contact of your choosing. Source Source 2
Here’s How It Works: When you make a call with SOS, your iPhone automatically calls the local emergency number. In some countries and regions, you might need to choose the service that you need.
You can also add emergency contacts. After an emergency call ends, your iPhone alerts your emergency contacts with a text message, unless you choose to cancel. Your iPhone sends them your current location, and, for a period of time after you enter SOS mode, it sends updates to your emergency contacts when your location changes.