Long gone are the days of Modular, analogy, digital or hybrid PABX
systems. With any computer and the Phone system software, you have a
full PABX.
We market two models of Phone systems namely
    3CX   and
download their literature or free software as you wish

3CX Phone System is a software-based IP PBX for Windows
which replaces traditional PBX hardware phones. It is based on
the SIP standard and supports standard SIP software / hardware
phones. It allows calls to be made via a VOIP service provider or
via the regular PSTN. Easy to install and configure, it includes a
web-based configuration interface, voice mail, auto attendant,
call hunting and other features. Both commercial and a free
edition are available.
Download for free here