How can I make an emergency call on Apple Watch?

Apple Watch SOS

Yes, the Apple Watch can automatically call emergency services in your area, and send your location to your emergency contacts. Press and hold the side button under the Digital Crown to activate SOS mode.

There are two requirements for SOS mode to call 911:

1. An iPhone must be paired with Apple Watch, or Apple Watch must be connected to Wi-Fi.
2. The Apple Watch must be running watchOS version 3 or later.