Field Test Mode

Some phones have secret menus built in that allow you to gain access to field test and engineering features. This can range anywhere from seeing raw power measurement and cell id, to controlling levels of the physical layer and mobility management in the handset. There are some awesome features, but many ways to get at these modes. Here's a great place to keep notes on this!

Smartphones

Android (general)

Dial: *#*#4636#*#*

(Features vary from handset model to handset model.)

iOS

Dial: *3001#12345#*