Too many people rely solely on their cell phone as their phone book, rather than memorizing a few key numbers. This is especially true for teenagers and young adults. If their phone is broken, flooded, lost, stolen or juiceless then they can't call you.
As you put together an "emergency phone check-in spot," take all your potential numbers and run them through What Does My Phone Number Spell? If a number makes an easy to remember word or phrase, then give than number a +3 on "likely to be used."
How do I know this? One art project at Burning Man has been a fake-looking but fully operational phone booth that allows free phone calls to anywhere in the world. (Here's a photo gallery of the phone in use. People are in all sorts of attire, if you want warnings about that.)
As I've hung out by it, a great many people have come up only to say "I'd love to call my sister in Japan / my friend in NYC / my parents in London... but I don't have my cell phone with me, so I don't have any numbers!" I've noticed that the probability of this goes up as their age goes down.
Ask your family and friends in your emergency contact circle which numbers they have memorized, and help them find ones that are easy for this.
