Asterisk - An open source telephony switching and private branch exchange service for Linux.
Callmama - Get cheap international calling, international virtual number, Call Mama Gives an added advantage to send message and make calls to non-user of Callmama App.
Wazo.io - The UCaaS Platform Built for MSPs
CallHippo - Easy to use Phone System with World-Class Support
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.
Numero eSIM - Numero eSIM is a phone-number app that makes voice and data possible anywhere in the world, for free.