Field test mode is a special mode that changes the cellular signal strength bars on your iPhone into numbers for more accurate antenna signal measurement. To see signal strength in dB instead of bars follow these instructions:
1. Your iPhone must be running iOS 4.1. Upgrade if necessary.
2. Open the Phone app.
3. Dial *3001#12345#* on the Keypad and touch Call.
4. Your iPhone is now in field test mode.
5. To exit press the home button.
Smaller negative numbers indicate a weaker signal (for example -110 is worse than -50).