There is no way to turn a phone truly "off", but you can isolate it from both listening in on conversations -- which modern phones do regardless of how they are

There is no way to turn a phone truly "off", but you can isolate it from both listening in on conversations -- which modern phones do regardless of how they are configured -- and preventing it from connecting to a network. You're never going to escape government surveillance, but you can take steps to prevent advertisers from hoovering up every last bit of personal data that they can.