Aterisk is a,
- Open source implementation of a PBX
- Runs on Windows/Linux/UNIX and performs all the features of a proprietary PBX,
- DID/DOD, voice mail, conference calling, interactive voice response (phone menus), and automatic call distribution.
- Runs on a standard desktop computer,
- Needs a special PCI card to attach to the PSTN or other analog phones (a modem will not work).
- Can also be used to run a VOIP network with connections to the PSTN
- With VOIP, this can be incredibly cheap.