Dialpad - Switch is a cloud-based phone system built for Google Apps users.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
3CX - Hosted PBX with CRM integration and mobile apps
Zoom - Equip your team with tools designed to collaborate, connect, and engage with teammates and customers, no matter where you’re located, all in one platform.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
Asterisk - An open source telephony switching and private branch exchange service for Linux.