Do you usually go abroad to a foreign country as part of your job requirements, especially when you travel to Canada or Europe? When you are there you may need to use your iPhone 5 via Wi-Fi; however, without incurring any data roaming charges or even any other cellular charges onto your phone bill.
Normally, you can head to Settings > General > Cellular, then switch OFF both Cellular Data and Roaming options on your phone. But there is another way to prevent all other charges. You will need to turn on Airplane Mode, then Turn Wi-Fi back on.
However, noted in mind that these actions can only allow you to receive email and surf the web on Wi-Fi. You will not be able to use Siri on iPhone 5 to make a telephone call, based on Wi-Fi network.