*#0*# – Enter test mode (only available on some devices).
*#*#225#*#* – Display Calendar data stored on your phone.
*#*#7594#*#* – Change power button behavior.
Keep in mind that if you continue with the listed codes, you will do it at your own risk.
If you are using an Android tablet that doesn’t have the dialer, you can find apps on Google Play Store that look for secret codes on your tablet.
Codes for Checking Firmware Version Info