Asterisk

Asterisk is an open-source PBX and telephony toolkit. It acts as middleware between internet and telephony channels on one side and telephony applications on the other. The project describes itself as supporting far more than internet telephony, with extensive support for traditional public telephone network connections as well. The codebase is written in C.

Most deployments today use VoIP and SIP exclusively, but Asterisk can also interface with TDM and analog services when the right hardware is installed. Supported telephony hardware includes Sangoma analog and digital interface cards, full-duplex sound cards supported by PortAudio, and the Xorcom Astribank channel bank. It is developed and tested primarily on GNU/Linux and reportedly also runs on macOS and the BSD variants.

Asterisk is built from source and needs a compatible compiler and development libraries, so it suits telephony engineers, service providers and administrators comfortable with that process. The project strongly urges new operators to read its security documentation before configuring a server, and its change logs are the starting point when upgrading between versions.