Unlock my old phone

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

You will need to contact the carrier the phone is currently locked to and ask them to unlock it.

How to unlock your iPhone for use with a different carrier - Apple Support