Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
qTox - A Qt graphical user interface for Tox.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Tox - Send messages, files, create groups and make Tox to Tox audio and video calls. Securely.
Plivo - Plivo simplifies your customer engagement.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.