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.
Slack - A messaging app for teams who see through the Earth!
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Microsoft Teams - Microsoft Teams provides the enterprise-level security, compliance and management features you expect from Office 365, including broad support for compliance standards, and eDiscovery and legal hold for channels, chats, and files.