When you're trying to make a call and there's just no signal around, the last thing you want to do is walk around the room holding your phone in strange and ludicrous positions, looking very silly in the process. Now, we can all rejoice, as there's no need to ever crouch down in the corner of a room or hang most of your body out of a window to find that elusive signal.
Just call *3001#12345#* to launch your iPhone's hidden Field Mode tool. This will turn your normal signal bar into an easier-to-understand numerical display. If your phone is hovering around the -50 mark, you'll be enjoying the best signal known to man. However, anything near -120 and you'll be struggling to do anything. Just follow the numbers to get to a better location.