Guarantee means nothing with Mtn

Mtn contacted us for an upgrade, phone was delivered 30 November. Had issues with the phone shutting down because of overheating but the phone does not even get hot. After visiting two mtn stories they advised the phone needs to be replaced but because I do not have the box they can’t do that! What guarantee is that? No one had ever told us to keep the box! Been a customer for more than 20 years! Not for much longer!!! Disappointing, after never having any issues before