iPhone 5s
Under the Field Test, users will find five main categories: MM Info, SIM Info, UMTS Cell Environment, GSM Cell Environment and PDP Context InfoGetty Images

Users who would like to access some hidden features on their iPhones can do so by entering a simple code and dialling the call button. Field Test allows users to change their phone's privacy settings.

Field Test can be enabled by dialling *3001#12345#* and pressing the call button. After you do this, the phone's screen will turn grey, revealing five main categories: MM Info, SIM Info, UMTS Cell Environment, GSM Cell Environment and PDP Context Info. Users can access these categories by pressing on the relevant tab.

The feature that is most engaging in the Field Test is the detailed signal strength indicator. It shows the exact signal strength the phone has at any given time. On entering the Field Test, iPhone users can immediately see signal strength in numerical value, visible at the top left hand corner of the iPhone's screen.

According to News, the scale ranges from -40 all the way through to -140. When the signal strength indicator shows anything above -80 it roughly equates to a full signal reception but anything below -110 means zero signal reception.

Users who would prefer to change the default reception signal — which is in dots in all iPhones — can do so by holding down the power button after entering the Field Test mode. This done, the message to power off the device appears. This is when users need to press the home button and the feature will be enabled.

Users can also view both ways of showing the reception strength — dots and numerical value — by simply tapping on the top left hand corner where the icon is displayed.

More code users can try:

The code *#33# will show the status of your call barring settings. Users can turn the call barring on and off by dialling *33*pin# and #33&pin#.

The code *#43# lets users know whether call waiting is enabled. Users can turn call waiting on and off by dialling *43# and #43#.

The code *#06# lets users check their iPhone's unique 15-digit IMEI identifier number. The IMEI identifier number is very useful as it allows users to remotely shut down their device when stolen.

The Field Test is functional on all iPhones.