Really, really hidden features that will only appear if you chant the correct spells:

Dial :

**3001#12345#* *

to bring up the spirit of iPhone's Field Test mode (this is different from the DFU mode familair to jailbreakers), which will give you information about towers and signal strength, 3G/EDGE network and much more info which no mortal would ever need.

*#06#

will display your IMEI. No need to tap Call.

And some spells that only works in AT&T land:

*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)

And a few more enchantments related to call forwarding :

*#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.

And lots and lots of other spells for all occassions, phew...

*#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 similar 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.

*#61# and tap Call.

Check the number for unanswered calls.

Show the number for voice call forwarding when a call is unanswered. Also show the options for data, fax, sms, sync, async, packet access and pad access.

*#62# and tap Call.

Check the number for call forwarding if no service is available.

Just like the previous, except for no-service rather than no-answer situations.

*#67# and tap Call.

Check the number for call forwarding when the iPhone is busy.

And again, but for when the iPhone is busy.

*#33# and tap Call.

Check for call control bars.

Check all the usual suspects (voice, data, fax, sms, etc) to see whether barring is enabled or disabled for outgoing.

originally submitted by GeneQ on Super User.