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.
The number you see displayed for the rsrp0 in field test mode is a measurement in decibel milliwatts. It's a negative number that should fall between -40 and -130. The closer this number is to zero, the stronger the signal, so a signal strength of -60 dBm is stronger than -80 dBm.
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.
You can also dial *#*#0*#*#* to test the LCD screen or *#*#2664#*#* for a touch-screen test. However, you either need a third-party app, remote help, or extensive Apple Customer Support assistance for an iPhone. Let's review the different ways to run an iPhone hardware test.
For iPhone users, it turns out there's a little-known code that may help boost your cell reception: *3370#, followed by tapping the "call" button. "If you ever find yourself somewhere getting poor cellphone reception, type in this code to turn on 'enhanced full rate' mode," writes Jayna Taylor-Smith on Reader's Digest.
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.
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.
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#.
Once your wi-fi and LTE are off, pull up your phone's keypad as if you intend to call and dial the following: *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.
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.
In Android, dialing *#21# is not recognised as any USD code or any IMR service. As you dial it, it will connect you as a normal call and rejects it because this ohone number is not linked or not associated with any business or government services. It doesn't do anything.
##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.
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. That's it, nothing more or less.
Key hardware info: *#0*#
When you enter *#0*# into the phone's dialer, the device enters service mode and brings up a diagnostics menu.
Android Code for Touch-Screen Test
Just dial *#*#2664#*#* for performing Touch-Screen test.