[beala]

On iPhone, you can quickly do this by holding down the lock button and either volume button until the shutdown screen appears. Once it appears, your phone is now locked and it will only accept the PIN (you don't need to actually shut down).

[minitoar]

Alternatively one can press the lock button 5 times quickly.

[aftbit]

On Android, pressing lock 5 times quickly automatically dials 911.

[TeMPOraL]

Thankfully, it doesn't. It asks you to confirm by sliding some on-screen control, and then dials 911 / 112.

If it dialed immediately, I'd be in jail already, going by the amount of times I managed to trigger the "call 911?" screen by accident in the last year or so.