Oppo India Noida - Denial of service during warrenty by OPPO India

My oppo enco m** Bluetooth headphone stopped working within few months after purchase. It was under ** year warrenty. When I visited the service center they are telling me to bring the box of the Bluetooth head set then only they will provide replacement as it cannot be repaired. I have submitted everything including original invoice to them but still they are not listening. I have gone through their guarantee and warranty policy nowhere it is written that box needs to be produced to claim warrenty. Simply cheating. I want to teach these guys a lesson.