There's a new Emergency Mode in iOS 11 that can call the local authorities if you need help. You don't need to unlock your phone or dial anything to use this.
Once setup, you just need to click the iPhone power button five times fast. This is not on by default, but if you turn it on you can configure the iPhone to call local authorities when you do this. You can set it up to call automatically, to sound an alert and you can set it up to text your emergency contacts.
Go to Settings -> Emergency SOS and choose the options you want enabled. To use it, just click the power button five times and it will activate and start a countdown. You can cancel it in 3 seconds.