PATCH /conversations/voice/{id} 403 when updating participant attributes

Trying to push custom attributes from our CRM into a live voice call. The PATCH request to /api/v2/conversations/calls/{conversationId}/participants/{participantId}/attributes returns a 403 Forbidden, even though the token has admin:conversation:write scope.

{
 "attributes": {"crmId": "999"}
}

Any idea why the scope isn’t enough?