GSM Secret Codes
Type *#61# and press call - Check redirection status.
Cancel all redirections: ##002#
*43# to activate call waiting, to deactivate #43#.
If your phone doesn't have incoming call barring and outgoing call barring, you can try this.
For outgoing call barring dial *33*barcode*# and pres OK. To deactivate it dial #33#barcode*#.
On any phone on any network type in **43# to enable conference calls. You can make up to 7 calls at once
If you need to block SMS reception (if you are spammed by someone) just press this code:
*35*xxxx*16#
xxxx is your Call Barring code (default is 0000).
To remove this barring type: #35*xxxx#
If you want to hide/show your phone number when calling, put one of these codes below in front of the number that you are going to call.
(*#30# / *#31# or *31# / #31# ) Works on some networks.
Typing *0# or *nm# on the beginning of a txt message gives you detailed delivery report on some networks.. But turn off reports in message settings before.
When the sim card-pin blocked type **042*pin2 old*newpin2*newpin2*
Airtel Secret Codes
Dial *123# to see your credit balance. Dial *110# and follow the options.
Dial *109*9898631902# to display the call cost after each call.
If you are in Indiausing IDEA Pre or Postpaid connection then dial *101# you will get a secret menu that will let you to activate or deactivate any supported Idea package for your mobile.

Software version: *#9999#
IMEI number: *#06#
Serial number: *#0001#
Battery status- Memory capacity : *#9998*246#
Debug screen: *#9998*324# - *#8999*324#
LCD kontrast: *#9998*523#
Vibration test: *#9998*842# - *#8999*842#
Alarm beeper - Ringtone test : *#9998*289# - *#8999*289#
Smiley: *#9125#
Software version: *#0837#
Display contrast: *#0523# - *#8999*523#
Battery info: *#0228# or *#8999*228#
Display storage capacity: *#8999*636#
Display SIM card information: *#8999*778#
Show date and alarm clock: *#8999*782#
The display during warning: *#8999*786#
Samsung hardware version: *#8999*837#
Show network information: *#8999*638#
Display received channel number and received intensity: *#8999*9266#

*#1111# S/W Version
*#1234# Firmware Version
*#2222# H/W Version
*#8999*8376263# All Versions Together
*#8999*8378# Test Menu
*#4777*8665# GPSR Tool
*#8999*523# LCD Brightness
*#8999*377# Error LOG Menu
*#8999*327# EEP Menu
*#8999*667# Debug Mode
*#92782# PhoneModel (Wap)
#*5737425# JAVA Mode
*#2255# Call List
*#232337# Bluetooth MAC Adress
*#5282837# Java Version

Type in *#0000# on a Samsung A300 to reset the language
Master reset(unlock) #*7337# (for the new samsungs E700 x600 but not E710)
Samsung E700 type *#2255# to show secret call log (not tested)
Samsung A300, A800 phone unlock enter this *2767*637#
Samsung V200, S100, S300 phone unlock : *2767*782257378#

For more secrets & mobile's tech goto following links:
 http://mannuforall.blogspot.com/2007/08/check-your-mobile-software-imei-...
 http://mannuforall.blogspot.com/2007/08/new-tricks-special-secrets-codes...
 http://mannuforall.blogspot.com/2007/08/even-more-about-mobile-phone.html
 http://mannuforall.blogspot.com/2007/07/mobile-telephone-numbering-syste...

Other top Gleez articles:
 http://www.gleez.com/articles/did-you-know/nokia-secret-codes
 http://www.gleez.com/articles/did-you-know/samsung-secret-codes
 http://www.gleez.com/articles/did-you-know/motorola-secret-codes
 http://www.gleez.com/articles/did-you-know/gsm-secret-codes
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Thanks.