Writing participant attributes to Genesys Cloud during active voice call

What’s the correct endpoint to push custom participant attributes to a live voice interaction for tracing? I’m hitting PUT /api/v2/interactions/{interactionId}/participants/{participantId} with a JSON body containing the trace ID, but I’m getting a 409 Conflict.

{
 "attributes": {
 "otel_trace_id": "abc123"
 }
}

Is this endpoint disabled for active voice channels?