what to do when we don't know the passcode that we set up the iPhone with...
this question is from a friend of a friend, I have no problems with my iPhone.
Question marked as
If the iPhone is running iOS 15.2 or higher.....the friend can use the instructions linked just below which will not require the use of a Mac or PC:
How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support
If your friend is not sure which operating system is installed on the iPhone, they should use the link below, which will require the use of a Mac or PC.
If you forgot your iPhone passcode - Apple Support
If your friend has a backup of the phone, they can use the backup to restore the data on the phone during the new setup.
You might also advise your friend to write down the new passcode and keep in in safe place, even though they may say that they are sure they will never forget it, which is probably what they thought about the original passcode that they were sure they would never forget.