iphone 5 power button does not work properly!

Hi!
I'm in Vietnam, I have bought my iPhone 5 from Viettel carrier for 2 months, recently I notice that my iPhone 5 power button does not work properly, it only can use in the left side, the right side of power button is unresposive. I took my iPhone 5 to Viettel carrier, but they refused my case 😟 I don't know how to report my case to Apple, this issue makes me feel very angry !!! They just told me brought my iPhone 5 back and tried to push the power button until it's totally broken. So anyone know how to solve my issue !? Where or who should I contact ?

Thank you!