ICE your cell phone! In this case, "ICE" stands for "In Case of Emergency".
If you were injured such that you could not communicate with emergency or rescue workers, your cell phone could provide vital contact information. It's simple—just enter the the letters "ICE" as if it were the name of a contact, and enter the phone number of your next of kin, or someone who could reliably provide information about you to emergency personnel. To provide a little more information for rescuers, include the name or other word to identify the person whom should be called, such as "ICE-Dad" or "ICE-Mary". Using this method, you could also include multiple ICE entries in case the first one doesn't answer.
You should make prior arrangements with the person you use as your ICE contact, or at least let them know, so that they will cooperate with emergency personnel. Ideally, the ICE person should be able to provide family contact information, your physician's name, and be aware of your medical history, allergies, and so on.
While ICE on your cell phone is one good way to provide emergency contact information, you should also have the same ICE contact information in your purse or wallet in case your cell phone is damaged or otherwise inaccessible at the time of the emergency.