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. For example, you can view a more precise display of your cellular signal strength and set up call barring to block outgoing phone calls.
Many interrogation codes do things you can now do from your iPhone’s normal Settings screen. All interrogation codes are used by opening the Phone app, typing a code into its keypad, and tapping the call button. 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 five dots. 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 “Call”.
You’ll see the numbers appear in the upper left-hand corner of your screen, as shown below.
You can set up “call barring”, preventing any outgoing calls until you disable the call barring feature. This feature is not available in your iPhone’s Settings screen, 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 Phone > 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 “Call”. Replace “PIN” with the numerical PIN of your SIM card. 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.
To disable car barring and allow outgoing calls, plug the following code into the dialer and tap “Call”. Replace “PIN” with your SIM card PIN, if you’ve set one. If you haven’t, you can type any number you want.
The dialer will accept any value if you haven’t set a PIN, so you could type
*33*0# to enable call barring and then type
#33*1# to disable it.
To check the call barring status, plug the following code into the dialer and call “Call”.
Less Important Codes
There are other codes, too, although they aren’t as frequently used. Many of these codes just provide another way to change settings and access information you can find on your iPhone’s Settings screens. Other codes are less important and provide access to information you probably don’t need.
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 > Phone > Show My Caller ID.
View IMEI Number: Type
*#06# to view the International Mobile Equipment Identity number 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 > 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 > 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.
SMS Message Center: Type
*#5005*7672# to view the phone number of your cellular carrier’s text message center. You probably will never need this number, but it may help with troubleshooting in some cases. You can usually also just ask your cellular provider for this number, if you need it.
There are other special codes you can type into your dialer, but they’re specific to different cellular carriers. For example, there’s probably a number you can dial to see how many minutes you have remaining if you have a limited number of minutes. Here are lists of codes for AT&T, Sprint, T-Mobile, and Verizon.