https://support.apple.com/en-us/HT208076
Here's how to make the call on iPhone 8 or later:
Press and hold the side button and one of the Volume buttons until the Emergency SOS slider appears.
Drag the Emergency SOS slider to call emergency services. If you continue to hold down the side button and Volume button, instead of dragging the slider, a countdown begins and an alert sounds. If you hold down the buttons until the countdown ends, your iPhone automatically calls emergency services.
Here's how to make the call on iPhone 7 or earlier:
Rapidly press the side (or top) button five times. The Emergency SOS slider will appear. (In India, you only need to press the button three times, then your iPhone automatically calls emergency services.)
Drag the Emergency SOS slider to call emergency services.
After the call ends, your iPhone sends your Emergency contacts a text message with your current location, unless you choose to cancel. If Location Services is off, it will temporarily turn on. If your location changes, your contacts will get an update, and you'll get a notification about 10 minutes later.
