Sometimes, iPhone 4 users are unable to connect because the weak signal strength and most of us blame Apple for its faulty Antenna design. But now, you can put your iPhone into field test mode, which will allow to test your cell signal reception quality. All you need is to update your iPhone 4, iPhone 3GS or iPhone 3G to iOS 4.1 and then Dial *3001#12345#* into your iPhone.
Eventually, the signal bars will be replaced with a number (like -75). The more negative the number, the worse the signal is. Example: -120 is worse than -75. Once activated, you can switch between signal bars and number by touching on the number.
If you’re on iOS 4.0.1 and jailbroken, then stay away from this update or you’ll lose your Jailbreak and unlock. However, iPhone 4, iPhone 3GS, iPhone 3G users on jailbroken iOS 4.0.1 or lower can install Signal Cydia App (from planetbeing) that displays the real signal strength, data about your cell reception and plots a map of your nearby cell towers. Signal iPhone app allow you to measure or check real signal strength that will eventually help you in taking a decision to switch to a different carrier or not.
