Imagine your mobile battery is very low. To activate, press the keys *3370#
Your mobile will restart with this reserve and the instrument will show a 50% increase in battery. This reserve will get charged when you charge your mobile next time.
THIS IS A HOAX! Unfortunately, this "tip" is totally bogus.
You cannot activate hidden or reserve battery power by keying in *3370# or any other code sequence.
The code '*3370#' can be used on some Nokia models to activate Enhanced Full Rate Codec (EFR).
Ironically, since this code activates the best sound quality on the phone, the change will actually reduce the length of time that the battery will last before recharging is required – in practical terms the complete opposite of what the tip suggests. Entering the code has no effect at all on brands of phone other than Nokia.
Source: http://www.hoax-slayer.com/mobile-phone-tips.shtml