*3001#12345#* followed by the Call button. This should take you to the field test display and give you several options such as your SIM info, serving cell measurements, and your connected mode. Once in this mode, follow the steps below to find the most relevant information to your signal strength.
Dial *3001#12345#* and press the Call button. This will launch the Field Test Mode app and where the bars/dots were in the top left corner of the screen, you'll now see a negative number. The negative number is the decibel signal strength reading and should be followed by the carrier name and then the network type.
Accessing Field Test Mode on an iPhone
Dial *3001#12345#*. This will launch the hidden Field Test Mode. Field Test Mode has two main tabs, All metrics, and Dashboard. In All metrics, you get to access different sections that display various types of data revolving around your cellular connection.
Hidden Battery Power / Boost Signal Strength
By pressing *3370# or #3370#, you'll activate your phone's “reserve battery power”.
To enter Field Test mode, open the Phone app and dial *3001#12345#* on the keypad. Yes, really. We weren't kidding about this being a truly hidden trick! Hit the call button, and Field Test will start up.
Check Your iPhone's Signal (*3001#12345#*) To check how much signal your iPhone has, enter Field Test Mode by typing *3001#12345#* on the keypad. This allows you to check all the details about your iPhone's signal, such as settings, network, and other technical data.
iPhone code: *3370#
If you ever find yourself somewhere getting poor cellphone reception, type in this code to turn on “enhanced full rate” mode. EFR mode improves your cell reception, but your battery life might be compromised.
*31# (Disable Caller ID)
If you want to keep your privacy and prevent others from knowing when you call them, you can disable caller ID by using this code. If you want to re-enable caller ID, just add the code again.
Key hardware info: *#0*#
When you enter *#0*# into the phone's dialer, the device enters service mode and brings up a diagnostics menu.
*2767*3855# – Format device to factory state (will delete everything on phone)
Check Call barring
Call barring lets you block incoming or outgoing calls to and from your phone. This may be useful if you keep receiving unwanted telemarketer calls or want to avoid expensive international calls. To check if the feature is enabled on your device, dial *#33#.
What does dialing *#21# actually do? When you dial *#21#, you are, in fact, activating a feature access code that quickly tells you the status of call forwarding for your phone. It's a quick way of seeing what, in the case of this particular 'setting interrogation code,' your call forwarding settings.
*#0*# - This will take you straight to the General Test Mode once dialed from the Phone app. It lets you test a bunch of things on your iPhone, such as vibration, touch, speaker etc. *5005*7672# - Dial this one up and you'll be presented with your SMS message center number.
Yes, *69 works on cell phones as well as landline phones. Dialing *69 will automatically call back the last number that called you. Keep in mind that this feature may not be available on all cell phone networks, and it may not work for calls that are blocked or private.
The *#9900# code is a diagnostic mode code for Samsung mobile devices. It can be used to access a variety of system information and diagnostic tools, such as enabling/disabling fast dormancy, logging options, and system dump mode.
Dialing ##002# deactivates any conditional or unconditional call forwarding settings on your account and also deletes any data such as messages or voicemails that were previously diverted to another number. This code only applies to phones on GSM networks such as AT&T or T-mobile.
Code to check if phone is tapped: *#21# Code to show unknown connections and tracking (for Android): *#*#4636#*#* or *#*#197328640#*#* Code to show unknown connections and if someone is tracing you (for iPhone): *3001#12345#*
Anonymous Call Rejection (*77) stops calls from people that use a blocking feature to keep their name or number from being displayed. When Anonymous Call Rejection is turned on, callers will hear a message telling them to hang up, turn off the blocking feature, and call again.
To program your Speed Calling 8 list, dial *74. Listen for the dialtone, then dial the one-digit number (2 through 9) you would like to use to dial a frequently called number. Then, dial the entire number you wish to designate for that code, followed by #. Remember to dial 1 and the area code for long-distance numbers.
When you do not want to be disturbed by incoming calls, you can activate the “Do Not Disturb” feature. All calls will be directed to Voice Mail and you will not hear your phone ring. To activate Do Not Disturb, dial *78.
For a Google Pixel, dial *#06# on your phone's keyboard to get the IMEI number. For phones with a removable battery, some IMEI numbers are listed on a sticker under the battery on the back of the phone.
Search for Network Activity
Another way to detect hidden cell phones is to search for network activity. Hidden cell phones can be used to send and receive data, just like any other phone. If you suspect that there may be a hidden cell phone in the area, try to connect to the local network.
##002# - If your voice call or data call, or SMS call has been forwarded, dialling this USSD code will erase them. *#21# - By dialling this USSD code, you would get to know if your calls have been diverted somewhere else or not.