8x8 - 8x8's VoIP business phone systems deliver affordable, cloud-based voice, video, messaging, and call center solutions, helping you serve customers anytime, anywhere.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Vonage - Vonage is a cloud-based all-in-one VoIP software solution designed for multiple industries such as accounting, healthcare, finance and marketing etc.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs