Twilio - Brings voice and messaging to your web and mobile applications.
DialogTech - DialogTech is an end-to-end call attribution and conversion platform.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Invoca - Call tracking & analytics software
BulkSMS - SMS messaging service provider offering two-way SMS communication services.
CallTrackingMetrics - Know who is calling and how they found you. Maximize the return on your advertising.