Tropo - Tropo is a cloud API for voice and messaging.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Melrose Labs SMSC Simulator - SMPP SMSC simulator to test SMS messaging applications.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs