There are some secret codes that can be used on iPhones to access hidden features or settings. Here are some of the most commonly used ones:

1. 3001#12345#: This code opens the Field Test Mode, which displays information about the iPhone's cellular signal strength and network connection.

2. *#06#: This code displays the iPhone's International Mobile Equipment Identity (IMEI) number, which is a unique identifier for the device.

3. *#31#: This code allows you to hide your phone number when making a call.

4. *#43#: This code checks if call waiting is enabled on your iPhone.

5. *#33#: This code displays the current call settings for the iPhone, including call barring, call forwarding, and call waiting.

6. *#76#: This code checks if the iPhone's Voice Over LTE (VoLTE) feature is enabled.

7. *#61#: This code displays the number of missed calls and the time until the next call diversion.

8. #50057672#: This code displays the SMS center number, which is used to route SMS messages to and from your iPhone.

9. *#43# + "Call" button: This code shows the status of call waiting on your iPhone.

10. *3370#: This code enables Enhanced Full Rate (EFR) mode, which improves the quality of voice calls on your iPhone. Note that this code may not work on all iPhone models.

11. *#21#: This code allows you to check call forwarding settings on your iPhone.

12. *#62#: This code checks if call forwarding is enabled on your iPhone when it's not reachable.

13. *#67#: This code checks if call forwarding is enabled on your iPhone when it's busy.

14. *#31# + phone number + "Call" button: This code allows you to temporarily hide your phone number when making a call to a specific person.

Again, it's important to use these codes with caution as they may not work on all iPhone models or with all carriers. Using them incorrectly could also cause unintended consequences such as disabling important features or settings on your device. It's always best to research the purpose of a code before attempting to use it.