Asterisk

Asterisk is an open-source telephony engine and PBX toolkit implemented primarily in C and developed for GNU/Linux. It exposes traditional PBX features and low-level telephony primitives so developers and operators can build SIP, WebRTC and PSTN-connected communications applications.

Key Features

• Modular, channel-based architecture with pluggable modules for SIP (chan_pjsip / chan_sip), media, codecs and hardware interfaces
• WebRTC support (WSS/DTLS-SRTP), RTP/RTCP handling, and modern codec support including Opus for browser and realtime audio
• ARI (Asterisk REST Interface) exposing REST + WebSocket events for building custom programmable call applications
• AMI and AGI interfaces for management, automation and traditional dialplan scripting; full CLI and menuselect build configuration
• PSTN and telephony hardware integration (traditional telephony cards and drivers) alongside VoIP gateway capability
• Source-driven build system using autoconf/Autotools and GNU Make; extensive documentation, community forum and release advisories

Use Cases

• Deploying an enterprise or branch PBX providing calls, voicemail, conferencing, queues and call routing
• Acting as a VoIP gateway or SBC to bridge SIP/WebRTC clients with PSTN trunks and telephony hardware
• Building programmable communications services (IVR, voicebots, conferencing, call recording) using ARI or AMI

Limitations and Considerations

• Nontrivial operational complexity: requires careful configuration, dependency management and familiarity with telephony concepts
• Requires proactive security and performance tuning (file-descriptor limits, TLS/DTLS configuration); security advisories are periodically published for critical fixes
• Feature surface is large and modularity means some functionality requires enabling/building specific modules or external libraries

Asterisk is a mature, widely adopted telephony engine suited for operators and developers who need deep control over call handling and media. It is maintained by a large community and is intended for production PBX and programmable-telephony deployments.