Twilio - Brings voice and messaging to your web and mobile applications.
Mobiniti - Mobiniti offers a mobile marketing platform for clients to invite customers to engage with their brand through SMS via their mobile phone.
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.
SendHub - Get a business phone system on your smartphone and computer. We provide professional, affordable sms, text, and voice solutions in an easy to use system. 14-day free trial.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
CallHub - Predictive Call Center, Voice and Text Broadcasting app