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The Mobile emergency number all over the world is 112.  It will work out of the coverage area of your mobile network and will search for an existing emergency number.  You can dial 112 even if the keypad is locked.

Unlock Your Car
Your cell phone will unlock your car if you have a remote unlock key at home. If you are locked out of your car call home, have someone hold the unlock on the key while holding the key to the home phone while you hold your cell phone close to the lock on the car door.  The car will unlock. You can be hundreds of miles away.

Hidden Battery Power
If your cell phone battery is very low, press *3370#.  This activates a reserve and your phone will show a 50% increase in battery, enough to get you to your charger.

Disabling Your Phone if Stolen
Record your phone's serial number by dialing *#06# and copying the 15 digits.  When your phone gets stolen, you can render it useless by calling your service provider and reporting the theft and giving your serial number.

Free Directory Service for Cells 
Cell phone companies are charging us $1.00 to $1.75 or more for 411 information calls when they don't have to. Most of us do not carry a telephone directory in our vehicle, which makes this situation even more of a problem. When you need to use the 411 information option, simply dial: (800)FREE-411, or (800) 373-3411 without incurring any charge at all.