Asterisk - An open source telephony switching and private branch exchange service for Linux.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Wazo.io - The UCaaS Platform Built for MSPs
MightyCall - Virtual phone system that lets you run your business from anywhere.
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.
Line2 - Line2 adds a second phone number or complete cloud business phone system to smartphones & computers that's robust yet affordable. Buy online or get a custom quote.