Invoca - Call tracking & analytics software
Asterisk - An open source telephony switching and private branch exchange service for Linux.
CallTrackingMetrics - Know who is calling and how they found you. Maximize the return on your advertising.
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.
Marchex - Marchex is a mobile advertising analytics company that connects online behavior to real-world, offline actions.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).