First of all, where did you buy your iPhone 6?
Second, if it is Japanese model iPhone you cannot turn off the camera shutter sound completely because of the Japanese law of the upskirt photography. The shutter sound will stay on the entire time even if you have lower the volume down or silent the ringer on the phone. I spoke to an Apple Technician one time about certified refurbished phones. I bought an iPhone 5 on Amazon.com back in July, but two times already they sent me two Japanese iPhones and not a US certified iPhone. I had to return them both because I was unsatisfied with the Japanese iPhones. Therefore, I ended up buying it on Ebay. Before buying an iPhone, I would suggest to ask the seller first if they carry another country's phone before purchasing the phone. Keep in mind, different countries have different laws such as video chatting and picture taking.
If you live in Japan, I would suggest you to just keep the phone because of the law in Japan when taking photos of anything, the shutter sound has to stay on. There is no way the Japanese phone can control in turning off the shutter sound completely. Due to the Japan law of the upskirt photography, I do not think it is not such good idea to buy another country's iPhone in which you want to turn off the camera shutter sound completely. In Japan, they prohibited you to turn off the camera shutter sound completely.