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).

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.

Asterisk 1.8 is a long term support version (i.e. it will be
supported for four years with an additional year of security only
fixes).  See:

https://wiki.asterisk.org/wiki/display/AST/Asterisk+Versions