Twilio - Brings voice and messaging to your web and mobile applications.
WikiPro - All-in-one Business to Customer Communication
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.
Respond Flow - The Mailchimp of SMS
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Proximiti MessagePro - MessagePro services include business SMS, secure messaging, interactive messaging, BOTs, scheduled messaging, and fax. Solutions for SMB and enterprise.