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 a Long Term Support version. It is scheduled to go to security fixes only on October 20th, 2024, and EOL on October 20th, 2025. See here for more information about Asterisk versions: https://docs.asterisk.org/About-the-Project/Asterisk-Versions/