Asterisk - An open source telephony switching and private branch exchange service for Linux.
Avaya Voice Portal - Voice Portal delivers on industry leading IT fit, reliability, scalability and manageability.
FreeSWITCH - FreeSWITCH is an open-source media application designed to support popular protools such as SIP and WebRTC and provides a platform to develop voice and video applications.
Scorebuddy - SO WHAT IS SCOREBUDDY ALL ABOUT? Years ago, physical score cards or computer spreadsheets were used to track customer interactions.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.