MTN refuses to hour warranty

I wish to make a complaint against MTN. Today my son and a friend went to the MTN shop in Cresta Shopping Centre. The reason they went there is because my sons Blackberry phone has started having problems and is still under warranty as it was bought last year in January. When my son and a friend went to the MTN store they were informed that they could not hand the phone in at the MTN shop as my son had lost his slip. It was pointed out to the woman working at MTN that the phone could be verified via it's IMEI number but the woman was not prepared to help and instead insisted that it was Jets problem as the phone was bought from there in the first place. They then went to Jet and explained what the woman at MTN had said to them and that although my son had lost the slip then phone should still be able to be booked in because of the fact that it could be traced via it