I've done some work in this area. We enable the caller to text but all of the data transmission done over the voice channel. This allows for locating the call, a live conversation and it could be made compatible with all 911 centers.

Our users are deaf but anyone could use this.

See http://nirmalpatel.com/research/deaf911.html

About time. Many people's lives have been saved by being able to send a text to a friend from somewhere in the backcountry / offshore where they couldn't make a call but there was enough signal for a txt.

havin hart @ack plz snd hlp xoxo

One obvious use is for when you have an intruder in your apartment.