This document discusses Asterisk, an open source telephony toolkit. It notes that Asterisk supports over 1 million production systems in over 170 countries using a dual license model. It can be used for IP PBXs, hybrid PBXs, VoIP gateways, and call centers. The document outlines Asterisk's modular architecture and APIs. It states that Asterisk 12 overhauled its architecture by introducing new channel drivers like PJSIP, a message bus called Stasis, and a new RESTful API called ARI.