Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Asterisk - An open source telephony switching and private branch exchange service for Linux.
TextMe - TextMe is a free calls and messaging application at both local and international levels.
Wazo.io - The UCaaS Platform Built for MSPs
Talkatone - Android App on. Google Play. Stop paying for cell minutes, texts & landlines; Turn your iPad or iPod touch into a phone; Call/Text even in areas with bad cell coverage; Call/Text when travelling without paying for expensive international minutes.
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.