MTN refuses service and refuses to honour warranty

On 06/05/15 MTN Gateway replaced a faulty Sony Xperia Z3 which they issued me in December 2014. The consultant who helped with the replacement informed that should the new phone show any faults within 7 days, I can return it to the store within 7 days at any time during office hours. On 13/05/15, the supposedly new phone stopped functioning. I went to the store at 17:15 (store closes at 18:00), I see the store closed with a sign on the door saying \closed for stock take". However there were 3 staff members on the store floor either standing around doing nothing or playing with their phones.I got 1 guys attention and asked for help and was ignored for a full 10 minutes. When he saw I wasn't leaving"