I (or rather my wife) recently had the issue described in this post
. Basically, she changed her SIM card while abroad and the SIM card lock kicked in. when she tried to unlock, no keyboard would appear. I then learned that it's because the phone needs a data connection before unlocking is possible. There was no explanation of this from the phone itself, and I had to ask the community here to get an explanation.
My suggestion is to include a text message in the lock screen in such a case, saying something along the lines of 'It isn't possible to unlock this device at this time because no network signal is available.'