16024: Invalid participant label, must not exceed 128 characters, must not be a CallSid, must not contain '/'

The participant label specified in an API call to create a voice conference participant was invalid. It may not exceed 128 characters, it may not be a CallSid, and it may not contain a '/' (even in URL encoded form %2F).

A POST to conference participants call to the Twilio API specified a participant label which is greater than 128 characters or which resembles a CallSid ("CA" followed by 32 lowercase hexadecimal characters), or contains the '/' character or URL encoded '/' character (%2F).

Shorten the length of the ParticipantLabel parameter, avoid using a call SID and avoid including '/' characters.