Mode: IMEI When you enter this number (no need to touch dial afterwards), you should be presented with your 15 digit IMEI number.
IMEI stands for International Mobile Equipment Identity.
The closer this number is to 0, the stronger the cell signal (or the closer you are to a tower).
tel://NUMBER will pop up a dialog asking if you wish to make a cellphone call to NUMBER using the baseband.
facetime://CONTACT will pop up a dialog asking if you wish to make a video call to CONTACT using Apple's Facetime over Wi-Fi.
If you’re interested in reading what exactly happens when you hold down the Volume Key, read on.
Most system tweaks depend on a framework called Mobile Substrate.
Similarly, IMEI, in combination with a SIM card, provides an easy method to track the location of a person.
Changing cellphone, you can be tracked by SIM card used.
As for the frequency you are using, use the following guide to match your ARFCN (Absolute Radio Frequency Channel Number): "Network Operating Mode", if it is NOM 001 then Voice has higher priority (Class A), while NOM 002 indicates Data has a higher priority than Voice (Class B). Note that this Field Test Mode was disabled in i OS 4.0, and re-enabled in i OS 4.2.1.
However, many of field test features listed are no longer available unless you use an earlier version of the firmware, or re-enable via homebrew.
In such situations the hard reboot solution also doesn’t work since the tweak tries to run again when i OS boots up.
To solve this, when your i OS device is booting up, simultaneously keep the Volume Up button pressed and you should be able to see your Lock screen again.
saurik had added this safe mode feature for Mobile Substrate last year to fix horribly broken jailbreak apps and tweaks.