Introduction - If you have any usage issues, please Google them yourself
The low level speech compression algorithm of the GSM suite is called GSM 06.10 RPE LTP (Regular Pulse Excitation Long Term Predictor). My colleague Dr. Carsten Bormann and I have implemented a GSM 06.10 RPE LTP coder and decoder in C. Its source code is freely available, and we encourage you to use it, play with it, and invent new real time media protocols and algorithms.
Our implementation consists of a C library and a stand alone program. Both are destined to be compiled and used on a Unix like environment with at least 32 bit integers, but others have ported it to VMS and a MS DOS 16 bit environment. GSM 06.10 is faster than code book lookup algorithms such as CELP, but by no means cheap to use it for real time communication, you will need at least a medium scale workstation.