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 standard version.  It went to security fixes only on
November 2nd, 2022, and EOL on November 2nd, 2023.  See here for more
information about Asterisk versions:
https://docs.asterisk.org/About-the-Project/Asterisk-Versions/