Asterisk is a complete PBX in software. It provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in three protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware. Asterisk provides Voicemail services with Directory, Call Conferencing, Interactive Voice Response, Call Queuing. It has support for three-way calling, caller ID services, ADSI, SIP and H.323 (as both client and gateway). This is an EOL long term support version. It went to security fixes only on 2020-10-24, and EOL on 2021-10-24. For more information about Asterisk versions, see https://docs.asterisk.org/About-the-Project/Asterisk-Versions/ NOTE: This version does not work with the zaptel drivers. It requires the newer DAHDI drivers which are still being ported. So, there is no hardware support available at this moment.