You could just wrap your phone in tinfoil, but there is an exact way that you have to do it. If you have ever been somewhere remote without signal, your phone will nuke it's battery trying to find signal by increasing the antenna's power. You have to put it in airplane mode to save your battery. Not matter what you do, even turning your phone off, the antenna will be live sending some signal. The takeaway is that even with a hacky Faraday cage, your phone will try it's hardest to phone home and could win with a tiny tear in the foil.