Line2 - Line2 adds a second phone number or complete cloud business phone system to smartphones & computers that's robust yet affordable. Buy online or get a custom quote.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
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.
8x8 Virtual Office - Use your iPhone, iPad or iPod touch to make and receive calls using your 8x8 Virtual Office phone...
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).