freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
Loop Communications - Loop Communications provides hosted VoIP business phone systems to small businesses and mid-sized 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.
Fonality - Fonality is a leading business communications software that adapts to your changing needs.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Vonage - Vonage is a cloud-based all-in-one VoIP software solution designed for multiple industries such as accounting, healthcare, finance and marketing etc.