http://www.cnet.com/news/text-to-911-what-you-need-to-know-faq/
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.
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.