why particularly over voice channel ? Since gsm voice is anyway highly compressed/ digitised its unlikely you will be able to use it the way you expect.
I try to transfer voice in data channel of GSM network but it has unfavorable results because of limitations of GSM network in data channel. I needed to transfer voice as data in online communication without delay and good quality. You believe that in GPRS network I can communicate online on data channel with good quality?
Are there commercial phone to transfer voice based on GPRS without compression. I need to apply some changes on voice, with new compression, and a commercial phone with a place to keep microcontroller board.