Twilio - Brings voice and messaging to your web and mobile applications.
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
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
CallHippo - Easy to use Phone System with World-Class Support
3CX - Hosted PBX with CRM integration and mobile apps