SIPfoundry sipX
The sipX solution is an open source Enterprise SIP PBX complete with voice mail, auto-attendant and lots of features.
It is suited for large and small Enterprise use as it supports fully redundant call control (HA) as well as a SOAP-based Web services interface. sipX offers plug & play Web-based management, including integrated management and configuration of attached phones and gateways. sipX is a modular server based solution that runs on standard Linux.
sipX does not require any additional hardware as it interoperates with any SIP compliant gateway, phone or application.
Pingtel, currently the main driver behind the sipX open source project, offers bundled versions of sipX with added features and functionality. Check out their website for more details.