In Singapore, the iPhone warranty are covered by the Singapore local Three majors telecommunication (Telecos) providers, which is SingTel, M1 and Starhub.
All iPhone sold with or without contract/plan from local telecos shops and apple premium resellers are covered by respective local telecos.
There are no such cases that the iPhone can be repair by Epicentre, iStudio, nubox and/or others major apple authorized premium resellers outlet.
If you get your iPhone from unauthorized dealers, please check with them if you could repair your iPhone from them or/and will there be any warranty covered before you decide to purchase it.
If you are SingTel customer, please bring your iPhone, transaction receipt/invoice, Simcard to SingTel Commcentre which is located at Somerset MRT. Website: http://www.singtel.com/
If you are M1 customer, please bring your iPhone, transaction receipt/invoice, Simcard to Paragon B1 or Plaza Singapura B1. Website: http://www.m1.com.sg/
If you are StarHub customer, please bring your iPhone, transaction receipt/invoice, Simcard to Plaza Singapura 1F. Website: http://www.starhub.com/
For more information, you can visit to apple Singapore website at http://www.apple.com.sg/ or tentatively you can call the apple Singapore hotline at 800-186-1087 or +65 6 835-1812