Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs