Estel, if you want FSK or PSK data over soundcard fldigi will do most of the work in most popular modes, for more network oriented traffic ax25 protocol will permit even tcp/ip over soundcard. These are well supported by OSS. These are both intended for low bandwidth radio so will easily fit in GSM voice channel. These are available for chroot debian on the repos right now. In the amateur radio repos there is even a very low bandwith app, WSJT, which averages tones over minutes for bouncing signals using the moon as the communication satellite, it is possible to connect worldwide using a 100w VHF or UHF amp, a preamp, a SSB radio, and a long yaggi made from grounding wire and a piece of wood or plastic pipe.