13240: Dial->Conference: Invalid Whisper SID

This error occurs when the Call SID supplied for conference whisper or the <Conference> coach attribute is invalid. Twilio requires coach to reference a call that is currently connected to an in-progress conference. If the specified call SID does not exist or is no longer connected to that conference, Twilio throws 13240 and the call fails to the action URL.

• The value passed for conference whisper or the <Conference> coach attribute is not a valid Call SID.
• The referenced call SID does not exist.
• The referenced call is no longer connected to the in-progress conference.
• The participant's call has already ended, the associated conference has ended, or the call was modified to use new TwiML, so it is no longer available as an active conference participant.

• Verify that the value used for conference whisper or coach is the Call SID of an active participant in the same in-progress conference.
• Confirm that the SID is a valid Call SID with the CA prefix and the expected 34-character SID format.
• Check the active conference participant list before using coach to ensure the target call is still connected to the conference.
• If the target participant has left the conference or the call has been redirected to new TwiML, use the current active participant's Call SID instead of a stale SID.

• TwiML Voice: <Conference>
• Conferences Participants subresource
• What is a String Identifier (SID)?