Twilio - Brings voice and messaging to your web and mobile applications.
Plivo - Plivo simplifies your customer engagement.
46elks - Adding SMS and voice to your application or service should be easy.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
HeyWire - Next-level customer service is just a text message away.