My understanding is that people with non flip-top phones tend to lock their keypads so that no one can make any calls without unlocking it to avoid making accidental 999/112 calls. From my memory of my sister's phone, when this lock is applied you can't access any of the info in the address book until you've unlocked it.

Yes, but that's usually "menu + *" or some other combination of buttons that's the same on all phones of that make rather than a number personal to you.