Evercontact - Your contacts always up to date and automatically with Evercontact.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
CONTACTBOX - CONTACTBOX combines the simplicity of an address book with effective functions of a CRM system.
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.
Bigin by Zoho CRM - Pipeline-centric CRM built and priced for small businesses
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.