TextNow - TextNow is a mobile virtual network operator (MVNO) that provides a free phone service to users by using other cellular networks coverage.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Wazo.io - The UCaaS Platform Built for MSPs
Burner - Disposable phone numbers -- now with MMS photo 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.