Twilio - Brings voice and messaging to your web and mobile applications.
Notix.co - NOTIX is an audience re-engagement service based on web and in-app push notifications that work for both desktop and mobile devices.
Plivo - Plivo simplifies your customer engagement.
Airship - Mobile marketing solutions across all major mobile platforms.
SendBird - SendBird is messaging-as-a-service.
OneSignal - Customer engagement platform used by over 1 million developers and marketers; the fastest and most reliable way to send mobile and web push notifications, in-app messages, emails, and SMS.