system environment/libraries
- dahdi-linux - The DAHDI project
- dahdi-tools - DAHDI Telephony Interface Driver
- libopenr2 - OpenR2 is a library that implements the MFC/R2 telephony signaling protocol over E1 lines
- libpri - An implementation of Primary Rate ISDN
- libresample - libresample, Real-time library interface
- libss7 - Libss7, an open source implementation of Signalling System 7 (SS7)