This secret iPhone code unlocks a hidden feature that may come in handy.. If you have an iPhone, there’s a secret code that you’ll want to know about, especially if your signal is not always 100%..! Secret iPhone hack: A step-by-step guide Everyone recognizes the five dots in the upper-left corner of the screen. The more black dots, the stronger the signal. But you can actually replace those dots with a more precise measurement: a number. First, open the phone app, enter the secret code *3001#12345#* and press the call button, which will bring you to Field Test mode. Once in Field Test mode, notice that the five dots in the upper-left corner have been replaced with a negative number. According to CNET , around -113 decibels is on the low end of the signal bar range and around -50 decibels is on the high end. In other words, your signal is stronger the closer it is to zero. In this case, -103 is strong enough to make calls, b...