Right at this moment, only subscribers in Black Hawk County to i wireless, a local carrier affiliated with T-Mobile, can use this service. Because text messages will not give a location of origin to those at 911 call centers, a reply text is sometimes necessary asking for the city or zip code where the emergency is happening. If the answer is a location within the county, the text message goes through to an operator who can send and receive text messages. Otherwise, the person with the emergency is sent a text urging him to call 911.
Intrado is also working with other call centers on upgrading to accept text messages. In the future, the company says that photos and videos will be accepted to summon help and that pictures or video can help responders know what to expect when responding to a call, and can also help identify possible suspects in a crime. Already there have been some cases in the U.S. where a kidnap victim was able to secretly text a message to summon help. The whole idea behind texting emergency calls is to speed up response time.
source: AP via EngadgetMobile