In a sticky situation that you need to get out of? Not sure if that date or meeting is going to go well? Maybe you should cover yourself and arrange for a call to come in, that'll let you excuse yourself to take care of some important matter? Maybe you need Getmooh.
Getmooh ("GETMeOutOfHere") is a free service that lets you phone yourself, after a manner of speaking. On their website, you can enter a number to call—most likely yours—a time to call, and a choice of messages to use. On their FAQ page, they warn that their system has some kind of bandwidth cap placed on it, such that if that capacity is exceeded, your call may not get made. I suppose there's really no such thing as a mission-critical "get me out of here", but if such a thing were to exist, you'd be well-served not to rely on these folks to guarantee that the call's going to be made.
You do need to sign up to take advantage of this service, and your phone number is included with all outgoing messages, so if you're planning to have a call made to somebody other than yourself, know that your recipient is going to know who's responsible for the call.