Verizon, AT&T, T-Mobile, and Sprint have come together to support texting 911, while there are plans to pencil in other carriers some time down the road. Emergency call centers are working on implement text messaging in their areas, where these centers will be controlled locally and not by the FCC.
In layman’s terms, mobile companies will be able to forward the texts, but the onus falls on the call centers to actually receive these emergency texts and to dispatch them. It is an exercise that will require tech upgrades as well as training, and all of these will take up time and resources. Text-to-911 does come in handy whenever one is in a dangerous or impossible to speak situation. However, do call whenever you can compared to text, as the inflection of your voice also reveals more about a particular situation than a plain SMS.