Josh Miller/CNET

Beginning May 15, wireless carriers in the US will uniformly and voluntarily support Text-to-911, a program that lets you send text messages to emergency services as an alternative to placing a phone call.

While carriers will climb on board, this just means they're making the service available -- the ability to text the police in an emergency situation won't work everywhere in the country the second May 15 rolls around. On the flipside, some counties have already embraced the program, usually working with a single carrier. Here are some important things to know about texting 911.

What is Text-to-911 and how does it work?

Text-to-911 is a free program for sending a text message addressed to "911" instead of placing a phone call. To use it, you address the message to 911 and enter the emergency in the body of the text, making sure that you also add your exact location -- or else emergency services won't be able to dispatch help your way.

Since it's all SMS-based, you will hear a response for more follow-up questions, or when help is on the way.

Who is Text-to-911 for?

Text-to-911 is useful for any situation in which it is dangerous or impossible to speak. Texting is also a useful way to help the younger demographic that feels more comfortable texting than calling.

Kent Hellebust, a vice president at TCS, a company that sells texting management software to emergency call centers, told CNET of an incident in which a ten-year-old girl was able to successfully get help by texting 911 -- apparently composing a text felt more automatic and natural than dialing in.

Which carriers support it?

By May 15, Verizon, AT&T, T-Mobile, and Sprint will support texting 911. Other carriers could also join in the future.

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Will it work where I live?

Although the carriers have committed to supporting 911 texting in their service areas, that doesn't mean that text-to-911 will be available everywhere. Emergency call centers, called PSAPs (Public Safety Answering Points), are the bodies in charge of implementing text messaging in their areas. These PSAPs are under the jurisdiction of their local states and counties, not the FCC, which governs the carriers.

In other words, it's up to the call centers to receive and dispatch your texts. Until the PSAP in your county first requests Text-to-911 support, implements the technology, and trains its staff, you won't be able to use texting in an emergency. However, some individual emergency services centers are ahead of the curve and already work with carriers to accept emergency texts.

How do I know if my county uses it?The FCC lists the states and counties currently using Text-to-911 (PDF).

What happens if my local call center isn't equipped to take my text?If your local 911 call center doesn't yet have the tools for text reporting, you'll receive a bounce-back text, so you know it hasn't gone through.

TeleCommunication System Inc. (TCS)

Do emergency texts receive priority?

That carriers treat SMS messages to 911 like any other text message, so your texts will be subject to the same service speeds or delays, depending on network strength in your area.

Does Text-to-911 replace regular 911 calls?

No; in fact, the FCC stresses that texting to 911 should be thought of only as a last resort, in the event that you can't speak. The FCC advises that people who are hard of hearing, deaf, or speech-impaired should still encouraged to use TTY for calling when they can.

Is it text-only? What about sending photos and video at a scene?

For the moment, Text-to-911 accepts written words only, though telecommunications agencies and companies are working toward Next-Generation 911, sometimes called NG911 for short. The goal is to modernize equipment and procedures at the emergency call centers (PSAPs), starting with photo messages and eventually including video messages as well. We're likely several years away from sending video messages to 911.

Who governs Text-to-911 exactly?

Text-to-911 is a program run by the FCC (Federal Communications Commission), in partnership with US carriers. Other telecommunications organizations -- like NENA, APCO, and the lobby group CTIA -- also have an interest in the program. That said, it's the individual counties and states that determine when they give texting the green flag, and how exactly they do it.