iPhone says unavailable on lockscreen
iPhone unavailable on lock screens
iPhone 6 Plus
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If you forgot the passcode on your iPhone, or your iPhone is disabled or unavailable -
How to reset your passcode without a computer if you have iOS 15.2 or higher.
How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support
How to reset your passcode using a computer.
If you forgot your iPhone passcode - Apple Support