Afilnet is one of the best SMS provider in the market, we have been working with this company for the last 10 years, and we're really happy with the service and support given.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Pony Express HQ - Pony Express HQ is an SMS text message marketing for small businesses, real estates, clubs, communities, dispensaries or non-profits. Send and receive text messages and images online with the large group of customers. Group texting app no reply all.
Twilio - Brings voice and messaging to your web and mobile applications.
Releans - Communication platform for Messaging and Chatbots
Atomic SMS Sender - Free bulk SMS software to send bulk text messages from PC
Clickatell - Simple, reliable mobile messaging cloud.