Here are some codes you can dial into the phone app that will access hidden information and menus in the iPhone:

*#06# – Displays IMEI

*3001#12345#* + “Call” – Accesses a hidden Field Test menu

*#43# + “Call” – Displays call waiting status

*43# + “Call” – Enables call waiting

#43# + “Call” – Disables call waiting

*#21# – Displays call forwarding status

##002# + “Call” – Disables all call forwarding

*33*pin# – Enables call barring

#33*pin# – Disables call barring

#31#phone-number + “Call” – Blocks caller ID for the current phone call

*3370# + “Call” – Enables “Enhanced Full Rate” and improves voice quality on GSM networks (may impact battery life)

*#5005*7672# + “Call” – Displays your carrier’s message center phone number