*3001#12345#* and tap Call. Enter Field Mode – Field mode shows the technical cellular specs of your phone.
*#06# Displays the IMEI – IMEI is the unique identifier your phone uses with the Cellular Provider. Together with the Sim information, you’re ready to get onto the providers 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)
*#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.