Asterisk - An open source telephony switching and private branch exchange service for Linux.
EspoCRM - EspoCRM is open source CRM (Customer Relationship Management) software that allows you to see, enter and evaluate all your company relationships regardless of the type.
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.
CiviCRM - A Constituent relationship management for the Civic Sector
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Less Annoying CRM - A simple customer relationship manager made just for small businesses. Manage contact info, track leads, and never miss a follow-up.