16026: Participant label is in use by another participant

The participant label specified in an API call to create a voice conference participant is already in use by another active participant in the same conference.

1. The specified label was already used in another API call to add a participant to the conference.
2. The specified label was already used in twiml (Dial Conference) to add an inbound participant to the conference

Use a label value which uniquely identifies this participant within the conference.