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.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
Cisco Webex Teams - Sign in to Webex Teams for group chat, video calling, and sharing documents with your team. It's all backed by Cisco security and reliability.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.