Notification Services (SMS/Push)
MagicBell
Real-time notification system with API & UI components
Twilio
SMS, push notifications, and VoIP APIs
Vonage
Video, SMS, chat, and voice APIs
Applozic
Chat SDKs, real-time messaging
Ably
Realtime messaging service with presence, persistence and guaranteed delivery. The free plan includes 3m messages per month, 100 peak connections, and 100 peak channels
knock.app
– Notifications infrastructure for developers. Send to multiple channels like in-app, email, SMS, Slack, and push with a single API call. The free plan includes 10,000 messages/mo
Novu
Open-source notification (Email, SMS, Direct, and Push) management. 
Stream
Feed and chat as a service