Not only do you have to explain why you were there but you might also have to come up with why you have a Nokia 3210 with credit in the glove box.
You have answered your question. Keep it in the glovebox as an "emergency phone". If you have a different sim inside of it than your regular phone this would support this too.
In some areas Three gets signal where EE doesn't and so forth.
Plus these elcheapo phones have batteries that last way longer than smart phones.
In terms of where I keep my punting phone. On my computer table in my bedroom as I also use it as a new post notification system on all the forums I go to (turn on email on every new reply/post) and add the email account to your phone so the phone makes a noise at every new post.
But get creative like that. Add a TV remote control app to your phone and use it as a TV remote. Get a Chromecast add a load of music to it use it as a media streamer to your TV through the chromecast.