Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Melrose Labs SMSC Simulator - SMPP SMSC simulator to test SMS messaging applications.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Plivo - Plivo simplifies your customer engagement.
Infobip - A2P messaging platform for enterprises, resellers & mobile operators with global network coverage.
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.