I had to clear my iphone of forwarding information and was not sure where what the code was.  So I did a bit of digging and found a list of them..

And I present them to you..  I have not tested them all but I will try a few and see what happens.

*3001#12345# and tap Call. Enter field mode
Field mode reveals many of the inner settings of your iphone
specially up-to-date network and cell information

*#06# Displays IMEI. No need to tap call
IMEI is the unique identifier for your cell phone hardware
together with your SIM information it identifies you to the
provider network

*777# and tap Call. Account balance for prepaid iPhone

*225# and tap Call. Bill Balance. (Postpaid only)

*646# and tap Call. Check minutes. (Postpaid only)
These three are pretty self explanatory

*#21# and tap call. Setting interrogation for call forwards.
Discover the settings for your call forwarding. You’ll see whether
you have voice, data, fax, sms, sync, async, packet access, and
pad access call forwarding enabled or disabled.

*#30# and tap Call. Calling line presentation check.
This displays whether you have enabled or disabled the presentation
of the calling line, presumably the number of the party placing the call.

*&76# and tap Call. Check whether the connected line presentation is enabled or not.
State whether the connected line presentation is enabled or disabled. Presumably similiar to the calling line presentation.

*#43# and tap Call. Determine if call waiting is enabled
Displays call waiting status for voice, data, fax, sms, sync data, async data,
packet access and pad access. Each item is either enabled or disabled.

Unconditional forwarding:

Forward service: All Calls
Activate: *21*phone_number#
Cancel & Deregister: ##21#
Cancel & Retain: #21#
Status: *#21#
Reestablish: *21#

Conditional forwarding:

Forward service: If Busy
Activate: *67*phone_number#
Cancel & Deregister: ##67#
Cancel & Retain: #67#
Status: *#67#
Reestablish: *67#

Forward service: If Not Answered
Activate: *61*phone_number#
Cancel & Deregister: ##61#
Cancel & Retain: #61#
Status: *#61#
Reestablish: *61#

Forward service: If Out of Reach
Activate: *62*phone_number#
Cancel & Deregister: ##62#
Cancel & Retain: #62#
Status: *#62#
Reestablish: *62#

*#21# Call Forwarding Overview

**61* destination number ** (Time 5 to 30 seconds) # divert after 5 to 30 seconds to ring
*61# Activate divert
#61# Call divert Disabled
##61# Deletes Diversion
*#61# Call divert status

**62* destination number # redirection in unattainability
*62# Turning
#62# Off
##62# Clear
*#62# Status

**67* destination number # for Divert if busy
*67# Turning
#67# Off
##67# Clear
*#67# Status

Simultaneous forwarding:

Forward service: All Forwards
Activate: *002*phone_number#
Cancel & Deregister: ##002#
Cancel & Retain: #002#
Status: *#002#
Reestablish: *002#

Forward service: All Conditional Forwards
Activate: *004*phone_number#
Cancel & Deregister: ##004#
Cancel & Retain: #004#
Status: *#004#
Reestablish: *004#
*#33# and tap Call. Check for call control bars
Here you will find all GSM-Codes/Secret-Code, for the iPhone are relevant.
*#06# Displays your IMEI on your iPhone.
*3001#12345#* To get your access to the field test mode of iPhone.
*#30# Thus, you can check whether the caller identification (Clip-function) in the iPhone is active.

*#43# Call Waiting Status
*43# Activate a waiting
#43# Deactivate waiting

Change PIN1: **04* old PIN * new PIN * new PIN #
Change PIN2: **042*old PIN * new PIN * new PIN #

*#746025625# SIM Clock Stop Mode
Whether your SIM card the SIM Clock Stop mode supported, you can find out with this code. The iPhone will then idle at the SIM card, ie no calls, etc., all activities on the SIM card. This has a positive effect on power consumption and prolongs battery life.

*3370# EFR mode
Enhanced Full Rate improves voice quality, but slightly reduced battery life.
#3370# ERF OFF mode

*4720# Half Rate (HFR) on
Opposite of ERA.
#4720# Half Rate (HFR) off

*21# Call divert a
#21# Call divert from
**21* destination number # number programmed
###21 deletes the call transfer is complete.

*31# phone number is transmitted
#31# phone number is not transmitted

*#5005*86# Are you the code, it displays the current mailbox settings.
*5005*86*# xxxxx Combox mail button on the available
(xxxxx = mailbox number)

*646# minutes consumption control

*135# Display Own Phone Number

*33* barring code # Outgoing calls to block the lock code to inquire about her please contact your network operator.
*#33 Unlock
*#33# Status

*331* barring code # Outgoing calls abroad to block the lock code to inquire about her please contact your network operator.
#331* barring code # lift blockade
*#331# status

*#5005*7672# SMS- center check
Current number of the SMS Center (SMSC) to find out. Each SMSC according to variable cost

*5005*7672*xxxxxxx # SMS Center reactivate
(xxxxxxx = SMS center number)