We need a system which can interract with a GSM mobile/modem and via audio channel to be able to send data (it's like a modem on a carrier).
The problem is that the GSM voice channel isn' a simple carrier - the audio is entered in a codec.
We need to transfer data with a bandwidth of 300-400 bytes/sec - actually the audio is at codec's input with resolution of 8000 frames/sec - 16 bit width.
The system we need must be on this voice carrier because in many countries the CSD line service isn't available (this service has a bandwidth of 9600bits/sec) - we need fewer BUT on the voice channel.
The GPRS also isn't a good solution for us because requires session establishing, the service isn;t so cheap in many countries/providers and especially at prepaired sim cards.
The final solution must be provided like a PC software which can send/receive data connected to a GSM phone/modem - 2 PC's and 2 phones which are involved in a voice call.
The connection with PC can be made through the soundcard or better via Bluetooth - on a SCO connection - like used in HFP profile.
I am an electronics engineer specialising in Digital Signal Processing. I could probably make the codec myself. Im presently working on an audio engineering project.