Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Keybase Chat - End-to-end encrypted chat built into Keybase
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs