MTN Failed to replace my device which stopped working before warranty expire

I bought an I Phone from MTN store, before it could complete a year it was giving me issues it couldn't switch on. MTN replaced my device with a refurbished phone which had an ear piece issues. I noticed this after setting up the device., I decided to take it back the next day. Tumi at Mall of Africa helped me to book the device in. it has been more than 2 months now waiting for the device they are calling me asking proof of purchase of this device. i don't know how are they expecting me to have a POP of a refurbished device. i regret taking back or purchasing in MTN