RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Kannel - Open Source WAP and SMS Gateway
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.