Twilio - Brings voice and messaging to your web and mobile applications.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Plivo - Plivo's Voice API enables businesses to build Voice apps to reach customers globally in 200+ countries. Try Plivo's Voice API for free. Instant Sign Up.
BulkSMS - SMS messaging service provider offering two-way SMS communication services.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
The SMS Works SMS API - A low cost and reliable SMS API for developers