Asterisk - An open source telephony switching and private branch exchange service for Linux.
Pipedrive - Sales pipeline software that gets you organized. Helps you focus on the right deals, so easy to use that salespeople just love it. Great for small teams.
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.
Zoho CRM - Omnichannel CRM for Businesses of all sizes
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Freshsales CRM - A full-fledged CRM for high-velocity sales teams.