I implemented this using UI Action + Flow Designer Subflow + Custom Action + Twilio integration
Usecase: ->A Major Incident is raised->An on-call coordinator clicks a “Send SMS” button->All members of the Assignment Group receive an SMS instantly.
Step 1: UI Action on Major Incident Form
visibility: active=true and major incident state = accepted
From UI action call subflow and pass incident record and assignment group
Step 2: Subflow
1. Retrieves all active users from the Assignment Group
2. Filters users with a valid mobile number
3. Loops through each user
4. Calls a custom Flow Designer Action to send SMS
Step3: Custom Action for Integrating with Twilio
I have attached all the screenshots
