Twilio - Brings voice and messaging to your web and mobile applications.
3CX - Hosted PBX with CRM integration and mobile apps
Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.
CallHub - Predictive Call Center, Voice and Text Broadcasting app
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Asterisk - An open source telephony switching and private branch exchange service for Linux.