Call Whisper with Selective Audio Controls

Nexmo's Selective Audio Controls allow supervisors to listen to calls without being heard by customers or agents. This feature uses participant IDs and conversation actions to control audio flow. A Node.js application is created using Express, which serves an answer URL that returns a valid NCCO based on the caller's role. The application generates three types of NCCOs: customer, agent, and supervisor, each with specific parameters to control who can speak and hear in the conversation. Once the application is complete, it uses a findParticipants method to search for leg UUIDs by role and stores them when a request is made to the answer URL.