I just realised I knew a tip that probably few of you know about.
From the locked screen, on a device with a PKB, type an emergency number (112 works for me, in the States maybe 911 does). From there you get prompted to dial an emergency number, so you can do it without unlocking the device. Very good to know - in these situations, seconds can make a difference.
Forgive me if this is mainstream knowledge.
Chris
Forgive my typos, I don't look when I type