Only the person that set the code can know that.
There is no default code. It needs to be setup by a person, and keyed in twice for it to take effect.
If you no longer remember the restrictions passcode, you will need to reset the iPhone to remove it.
From:
Use parental controls on your iPhone, iPad, and iPod touch - Apple Support
If you lose or forget your Restrictions passcode, you need to erase your device, then set it up as a new device to remove the Restrictions passcode.