Your iPhone has secret codes you can plug into the dialer to access hidden options. These codes “interrogate” the phone to find and change various settings or provide information such as your cellular signal strength. Here’s what you can do with them.

Field Test Mode

The most commonly used option here is probably Field Test Mode. Field Test Mode shows you more detailed information about your cellular signal strength, including a precise numerical value for your signal strength rather than the usual bars. You can walk around your home or office and see where your signal is strongest and where it’s weakest, for example.

To access Field Test Mode, open the Phone app, type the following code into the keypad, and tap the call icon:

 *3001#12345#*

The FTM Dashboard screen should appear with details about your carrier and your cellular signal.

Call Barring

You can set up “call barring” to prevent any outgoing calls until you disable it. This feature is not available in your iPhone’s settings, so you have to use these hidden codes to enable it.

You don’t need to set a SIM card PIN to use this feature. However, if you have enabled a SIM card PIN at Settings > Mobile Data > SIM PIN, you’ll need to know it. This is different from your screen unlock PIN.

To enable car barring and prevent outgoing calls, plug the following code into the dialer and tap the call icon. Replace “PIN” with the numerical PIN of your SIM card.

 *33*PIN#If you don't have a SIM card PIN, you can type any number you want in place of the PIN. The number you choose doesn't matter and can be a single digit if you want.

To disable car barring and allow outgoing calls, repeat the process above using the same code. You don’t need to use the same PIN if you don’t have one set. Any number will do.

If you can’t remember if you turned call barring on or off, you could try making a call. However, if call barring is off, your call may go through when you don’t want it to. To check the call barring status, plug the following code into the dialer and tap the call icon.

 *#33#

Other Useful Codes

There are other codes, too, although they aren’t as frequently used. Many of these codes provide another way to change settings and access information you can find on your iPhone’s Settings screens.

Anonymize Outgoing Calls: Type *#31# to view whether you have disabled caller ID and are making calls anonymously. You can also make a single anonymous call by typing #31#1234567890, replacing 1234567890 with the phone number you want to call.

Or, you can hide your caller ID for all outgoing calls by heading to Settings > Apps Phone > Show My Caller ID.

View IMEI Number: Type *#06# to view the International Mobile Equipment Identity number (IMEI) of your phone. This number uniquely identifies your phone’s hardware on cellular networks. It’s also visible at Settings > General > About.

Call Waiting: Type *#43# to view whether call waiting is enabled or not, type *43# to enable call waiting, or type #43# to disable call waiting. You can also view call waiting status and enable or disable it from Settings > Apps > Phone > Call Waiting.

Call Forwarding: Type *#21# to view whether call forwarding is enabled or type ##002# to disable call forwarding. You can also view call forwarding status and enable it from Settings > Apps > Phone > Call Forwarding.

Calling Line Presentation: Type *#30# to view whether your iPhone will display the caller’s phone number when an incoming call arrives on your phone. You can also tell whether this is enabled by whether a phone number appears on your iPhone when someone calls you.

There are other special codes you can type into your dialer, but they’re specific to different cellular carriers. For example, there may be a number you can dial to see how many minutes you have remaining if you have a limited call plan. Check out codes for AT&T, T-Mobile, and Verizon.

Interrogation codes for your iPhone allow you to find information or change settings that aren’t directly accessible through your iPhone settings. If you want to check your cellular signal or enable call barring, for example, interrogation codes are the easiest way to do so.

You may not use them often, but they’re useful to know about if the need ever arises.