DeskAlerts - Send alerts to any corporate device: PCs, phones, tablets, etc.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Notifuse - A 1-to-many notification API that's simple to use
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Text Alerts - Mobile Text Alerts allows administrators to send out mass SMS text messages to groups, it is useful way to keep group members informed about quick announcements.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs