Unified Communications Systems

• Microsoft Lync

Microsoft Lync Server (previously Microsoft Office Communications Server) is an enterprise real-time communications server, providing the infrastructure for enterprise instant messaging, presence, file transfer, peer-to-peer and multiparty voice and video calling, ad hoc and structured conferences (audio, video and web) and, through a 3rd party gateway or SIP trunk, PSTN connectivity.
http://en.wikipedia.org/wiki/Microsoft_Lync_Server
http://lync.microsoft.com/en-us/Pages/default.aspx