Asterisk

Asterisk as a PBX Asterisk was originally created as the engine for a PBX system and includes all of the components necessary to build a strong, scalable business phone system. These include advanced features that usually cost extra on a commercial phone system: things like voicemail, automated attendant, call queueing, conference bridging, parking, paging and intercom calling. Asterisk is technology and protocol agnostic, which means that you can connect it to the outside world using VoIP or traditional telephony technologies. It also means that you can use virtually any standards-based IP phone; Asterisk includes drivers for SIP and other protocols. That being said, Asterisk is future-proof. Unlike traditional phone systems that are generally upgraded using a forklift, Asterisk continues to evolve. Phone systems based on Asterisk see significant improvements each year as new features are included.