Disappointed

My husband and I got upgrades on our contracts, decided to go with the phones we like ( not cheap at all ). I had my phone all of 5 days when it started making flashing lines at the top and bottom of the screen when I am on whatsapp. Book the phone in through the store we got them from...... 2 weeks later I receive my phone back after so many calls and whatsapp messages to the store..... they "tested" the phone and found nothing wrong. I set the phone up... 5 mins in it starts doing the same thing. Mind you its only on watsapp so apparently they don't see it as a big deal. I did not select a cheap phone and I feel I should not have to sit with a faulty phone for 3 years because they don't feel like doing their jobs.