Twilio - Brings voice and messaging to your web and mobile applications.
Invoca - Call tracking & analytics software
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
CallTrackingMetrics - Know who is calling and how they found you. Maximize the return on your advertising.
BulkSMS - SMS messaging service provider offering two-way SMS communication services.
Marchex - Marchex is a mobile advertising analytics company that connects online behavior to real-world, offline actions.