3CX - Hosted PBX with CRM integration and mobile apps
Asterisk - An open source telephony switching and private branch exchange service for Linux.
CallHub - Predictive Call Center, Voice and Text Broadcasting app
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.
Kamailio - Kamailio® (successor of former OpenSER and SER) is an open source SIP server released under...
CallHippo - Easy to use Phone System with World-Class Support