If you'd like to see your actual signal strength (in dBm) on your iPhone instead of the normal little signal strength baubles, this is possible (though it's a bit of a hack).

1. Enter Field Test Mode

Open up the Phone app and dial in *3001#12345#* , then hit Call .

Field Test Mode will open up immediately and you'll see the signal strength in dBm in the upper left corner of the screen. Tapping the number (or baubles) will toggle between dBm and little baubles.

2. Make it stick

If you hit the home button and return to the home screen, the baubles will return, so don't do that.

To make the signal strength stick:

Hold the power button until the "Slide to power off" overlay appears, then: Release the power button and then hold the home button until the Field Test Mode quits

The signal strength should remain visible in the upper left corner of your screen, though I had to go through the whole procedure a few times before I got this to work (the first couple of times Field Test Mode just restarted instead of quitting).

What do the number mean?