Converting voice message to a text signal

I am interested in making an application for the deaf wherein the voice message at the caller end will be transmitted in the form of data packets such that they appear in the form of text messages at the reciever's end. As the reciever is a deaf person, he would use an instrument which has a LED that glows when he recieves a voice message.(instead of a ringtone). His reply to the same would be in the form of a text message which would be converted to voice at the other end.

Tips :

You can draw some idea from how a printer works. I mean it prints a prestored data. ADC can convert your voice into digital data and than try to do some hit and trial but intelligently.