The Federal Communications Commission has a guide on how to text 9-1-1 — but it will only work for customers on AT&T, Sprint, Verizon and T-Mobile as of now. Basically, if you can, call first. And if you get a bounceback text message saying that your text didn’t work, well, you’ll need to find a way to call.
If you have an emergency and live in one of these areas currently served by the program, you can send a message with your physical location and emergency to 9-1-1. The location is especially important as dispatchers can’t tell your location from a cellphone call.
Eventually, photos will also be supported in the service, though that also isn’t working everywhere yet.
Under the FCC’s new rules, all covered text providers must support text-to-911 by Dec. 31, 2014, but for now you can check this list [PDF] to see if your area made the cut.