16028: Participant to be whispered is not present in the conference

This error occurs when you try to add a coaching participant to a conference, but the participant you want the coach to whisper to is not currently connected to that active conference. Twilio only allows coach and call_sid_to_coach when the target Call SID is already present in the in-progress conference.

• The Call SID passed in coach or call_sid_to_coach does not belong to a participant who is currently connected to the in-progress conference.
• The participant has not joined the conference yet, or the participant already left and the call status is no longer active. Participant status can move through queued, connecting, ringing, connected, complete, or failed.
• The request is targeting the wrong conference, or the conference is no longer active. The Participants subresource only works with active conferences and active participants.

• Confirm that the participant you want to coach is already connected to the same active conference before you set coach or call_sid_to_coach.
• Retrieve the participant from the conference, or list the conference participants, and verify that the target Call SID is present in that conference and has an active status.
• Verify that you are using the correct conference identifier and the correct participant Call SID for the leg that is actually in the conference. Each call connection has its own Call SID. (help.twilio.com)
• If the participant disconnected, add or reconnect that participant first and then retry adding the coach. Use statusCallback events on the conference if you need to track when participants join or leave before starting coaching.

• TwiML <Conference>
• Conferences Participants subresource
• Voice Conference overview