Line2 - Line2 adds a second phone number or complete cloud business phone system to smartphones & computers that's robust yet affordable. Buy online or get a custom quote.
Twilio - Brings voice and messaging to your web and mobile applications.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Plivo - Plivo simplifies your customer engagement.
8x8 Virtual Office - Use your iPhone, iPad or iPod touch to make and receive calls using your 8x8 Virtual Office phone...
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates