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, 20. It is scheduled to go to
security fixes only on 2026-10-19, and EOL on 2027-10-19. For more
information about Asterisk versions, see
https://docs.asterisk.org/About-the-Project/Asterisk-Versions/