Twilio - Brings voice and messaging to your web and mobile applications.

DialMyCalls - The easy to use mass and emergency notification platform. Send a phone call, text message or e-mail to your entire list of people in seconds.

Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates

Call-Em-All - Call or send messages to friends and family members as well as other people who are online. The app allows you to create a message and send it out to multiple devices and pages.

BulkSMS - SMS messaging service provider offering two-way SMS communication services.

InformaCast - InformaCast is a leading mass notification software solution that helps organizations reach everyone with critical information.