Where do you enter the code for extra battery life on your iphone?

There is a code you can enter on your iphone which gives you extra battery life (*3370#) but where do you enter it?
No funny answers like its not true cus i know for a fact it is!

No there isn't. You're obviously a technophobe. The battery is a piece of hardware. It works by charging the Lithium atoms so that they are ions. Hence, "Lithium-ion" battery. The charge in the ions is then used as electrical energy to power the phone. You wouldn't be able to increase how much charge you could have by entering a few random characters, numbers and symbols.

The code you reference will actually result in decreased phone time. Read the page listed below to read the truth.