Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Kannel - Open Source WAP and SMS Gateway
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.
Plivo - Plivo simplifies your customer engagement.