Asterisk - An open source telephony switching and private branch exchange service for Linux.
TextNow - TextNow is a mobile virtual network operator (MVNO) that provides a free phone service to users by using other cellular networks coverage.
Wazo.io - The UCaaS Platform Built for MSPs
Dingtone - Dingtone is a free call and texting application for making unlimited calls and text messages to any number at the moment registered in any part of the world by any cellular company.
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.
TextMe - TextMe is a free calls and messaging application at both local and international levels.