*3001#12345#* and tap Call. Enter Field Mode.
Field mode reveals many of the inner settings of your iPhone, specifically up-to-date network and cell information.
*#06# Displays your 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 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.